Positive electrode active material particle and manufacturing method of positive electrode active material particle

ABSTRACT

Provided is a positive electrode active material which suppresses a reduction in capacity due to charge and discharge cycles when used in a lithium ion secondary battery. A covering layer is formed by segregation on a superficial portion of the positive electrode active material. The positive electrode active material includes a first region and a second region. The first region exists in an inner portion of the positive electrode active material. The second region exists in a superficial portion of the positive electrode active material and part of the inner portion thereof. The first region includes lithium, a transition metal, and oxygen. The second region includes magnesium, fluorine, and oxygen.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/702,862, filed Dec. 4, 2019, now pending, which is a continuation ofU.S. application Ser. No. 15/726,520, filed Oct. 6, 2017, now U.S. Pat.No. 11,094,927, which claims the benefit of foreign priorityapplications filed in Japan as Serial No. 2017-100619 on May 22, 2017,Serial No. 2017-052309 on Mar. 17, 2017, and Serial No. 2016-200835 onOct. 12, 2016, all of which are incorporated by reference.

TECHNICAL FIELD

One embodiment of the present invention relates to an object, a method,or a manufacturing method. The present invention relates to a process, amachine, manufacture, or a composition of matter. One embodiment of thepresent invention relates to a semiconductor device, a display device, alight-emitting device, a power storage device, a lighting device, anelectronic device, or a manufacturing method thereof. In particular, oneembodiment of the present invention relates to a positive electrodeactive material that can be used in a secondary battery, a secondarybattery, and an electronic device including a secondary battery.

In this specification, the power storage device is a collective termdescribing units and devices having a power storage function. Forexample, a storage battery such as a lithium-ion secondary battery (alsoreferred to as secondary battery), a lithium-ion capacitor, and anelectric double layer capacitor are included in the category of thepower storage device.

Note that electronic devices in this specification mean all devicesincluding power storage devices, and electro-optical devices includingpower storage devices, information terminal devices including powerstorage devices, and the like are all electronic devices.

BACKGROUND ART

In recent years, a variety of power storage devices such as lithium-ionsecondary batteries, lithium-ion capacitors, and air batteries have beenactively developed. In particular, a demand for lithium-ion secondarybatteries with high output and high capacity has rapidly grown with thedevelopment of the semiconductor industry, for portable informationterminals such as mobile phones, smartphones, and laptop computers,portable music players, and digital cameras; medical equipment;next-generation clean energy vehicles such as hybrid electric vehicles(HEV), electric vehicles (EV), and plug-in hybrid electric vehicles(PHEV); and the like. The lithium-ion secondary batteries are essentialas rechargeable energy supply sources for today's information society.

The performance currently required for lithium-ion secondary batteriesincludes increased capacity, improved cycle characteristics, safeoperation under a variety of environments, and longer-term reliability.

It is known to increase the capacity of a lithium ion secondary batteryby increasing the charging voltage. For example, the capacity of lithiumcobalt oxide, which is often used as a positive electrode activematerial of a lithium ion secondary battery, is 155 mAh/g when thecharging voltage is 4.3 V, and is 220 mAh/g when the charging voltage isincreased to 4.6 V (see FIG. 21A).

However, it is known that the cycle characteristics deteriorate due tothe increased charging voltage. For example, in general, the capacityretention percentage of lithium cobalt oxide is 95% or more after 30cycles when the charging voltage is 4.4 V, but is decreased to 50% orless after 30 cycles when the charging voltage is increased to 4.6 V(see FIG. 21B).

Thus, improvement of a positive electrode active material has beenstudied to increase the cycle characteristics and the capacity of thelithium ion secondary battery (Patent Document 1 and Patent Document 2).

Patent Document

[Patent Document 1] Japanese Published Patent Application No.2012-018914

[Patent Document 2] Japanese Published Patent Application No.2016-076454

DISCLOSURE OF INVENTION

That is, development of lithium ion secondary batteries and positiveelectrode active materials used therein is susceptible to improvement interms of capacity, cycle characteristics, charge and dischargecharacteristics, reliability, safety, cost, and the like.

An object of one embodiment of the present invention is to provide apositive electrode active material which suppresses a reduction incapacity due to charge and discharge cycles when used in a lithium ionsecondary battery. Another object of one embodiment of the presentinvention is to provide a high-capacity secondary battery. Anotherobject of one embodiment of the present invention is to provide asecondary battery with excellent charge and discharge characteristics.Another object of one embodiment of the present invention is to providea highly safe or highly reliable secondary battery.

Another object of one embodiment of the present invention is to providea novel material, active material, storage device, or a manufacturingmethod thereof.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects can be derived fromthe description of the specification, the drawings, and the claims.

In order to achieve the objects, one embodiment of the present inventionis characterized in that a covering layer is formed on a superficialportion of a positive electrode active material by segregation.

One embodiment of the present invention is a positive electrode activematerial. The positive electrode active material includes a first regionand a second region. The first region exists in an inner portion of thepositive electrode active material. The second region exists in asuperficial portion of the positive electrode active material and partof the inner portion of the positive electrode active material. Thefirst region includes lithium, a transition metal, and oxygen. Thesecond region includes magnesium, fluorine, and oxygen.

One embodiment of the present invention is a positive electrode activematerial. The positive electrode active material includes lithium, atransition metal, oxygen, magnesium, and fluorine. The total atom amountof the lithium, the transition metal, the oxygen, the magnesium, and thefluorine on a surface of the positive electrode active material that ismeasured by X-ray photoelectron spectroscopy is taken as 100 atomic %. Aconcentration of the magnesium on the surface of the positive electrodeactive material that is measured by X-ray photoelectron spectroscopy is1 atomic % or more and 16 atomic % or less. A concentration of thefluorine on the surface of the positive electrode active material thatis measured by X-ray photoelectron spectroscopy is 0.2 atomic % or moreand 4 atomic % or less.

One embodiment of the present invention is a positive electrode activematerial. The positive electrode active material includes lithium, atransition metal, oxygen, magnesium, and fluorine. A ratio between aconcentration of magnesium and fluorine on a surface of the positiveelectrode active material that is measured by X-ray photoelectronspectroscopy is Mg:F=y:1 (3≤y≤5).

One embodiment of the present invention is a positive electrode activematerial. The positive electrode active material includes lithium, atransition metal, oxygen, magnesium, and fluorine. A peak position of abonding energy of the fluorine on a surface of the positive electrodeactive material that is measured by X-ray photoelectron spectroscopy is682 eV or more and less than 685 eV.

In the above, the transition metal preferably includes cobalt.Alternatively, the transition metal preferably includes manganese,cobalt, and nickel.

One embodiment of the present invention is a positive electrode activematerial including a first region and a second region. The first regionexists in an inner portion of the positive electrode active material.The first region includes lithium, a transition metal, and oxygen. Thefirst region has a layered rock-salt crystal structure. The secondregion exists in a superficial portion of the positive electrode activematerial and part of the inner portion of the positive electrode activematerial. The second region includes magnesium, fluorine, and oxygen.The second region has a rock-salt crystal structure. Orientations of thecrystal in the first region and the crystal in the second region arealigned with each other. A ratio between a concentration of themagnesium and the fluorine on a surface of the positive electrode activematerial that is measured by X-ray photoelectron spectroscopy isMg:F=y:1 (3≤y≤5).

In the above, a peak position of a bonding energy of the fluorine on asurface of the positive electrode active material that is measured byX-ray photoelectron spectroscopy is preferably 682 eV or more and lessthan 685 eV.

One embodiment of the present invention is a manufacturing method of apositive electrode active material including a step of mixing a lithiumsource, a transition metal source, a magnesium source, and a fluorinesource, a heating step at 800° C. or higher and 1100° C. or lower for 2hours or more and 20 hours or less, and a heating step in anoxygen-containing atmosphere at 500° C. or higher and 1200° C. or lowerfor a retention time of 50 hours or less. An atomic ratio between thefluorine contained in the fluorine source and the magnesium contained inthe magnesium source is Mg:F=1:x (1.5≤x≤4).

One embodiment of the present invention is a positive electrode activematerial including a first region and a second region. The first regionexists in an inner portion of the positive electrode active material.The first region includes lithium, cobalt, and oxygen. The second regionincludes cobalt, magnesium, fluorine, and oxygen. L₃/L₂ of the cobaltincluded in the first region is less than 3.8 and L₃/L₂ of the cobaltincluded in the second region is 3.8 or more when the positive electrodeactive material is measured by electron energy loss spectroscopy.

According to one embodiment of the present invention, a positiveelectrode active material which suppresses a reduction in capacity dueto charge and discharge cycles when used in a lithium ion secondarybattery is provided. A lithium secondary battery with high capacity canbe provided. A secondary battery with excellent charge and dischargecharacteristics can be provided. A highly safe or highly reliablesecondary battery can be provided. A novel material, active material,storage device, or formation method thereof can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C show an example of a positive electrode active material.

FIG. 2 shows an example of a manufacturing method of the positiveelectrode active material.

FIGS. 3A and 3B are cross-sectional views of an active material layercontaining a graphene compound as a conductive additive.

FIGS. 4A and 4B show a coin-type secondary battery.

FIGS. 5A to 5D show a cylindrical secondary battery.

FIGS. 6A and 6B illustrate an example of a secondary battery.

FIGS. 7A1 to 7B2 illustrate an example of a secondary battery.

FIGS. 8A and 8B illustrate an example of a secondary battery.

FIG. 9 illustrates an example of a secondary battery.

FIGS. 10A to 10C illustrate a laminated secondary battery.

FIGS. 11A and 11B illustrate a laminated secondary battery.

FIG. 12 is an external view of a secondary battery.

FIG. 13 is an external view of a secondary battery.

FIGS. 14A to 14C illustrate a manufacturing method of a secondarybattery.

FIGS. 15A and 15D illustrate a bendable secondary battery.

FIGS. 16A and 16B illustrate a bendable secondary battery.

FIGS. 17A to 17G illustrate an example of an electronic device.

FIGS. 18A to 18C illustrate an example of an electronic device.

FIG. 19 illustrates an example of an electronic device.

FIGS. 20A to 20C illustrate an example of an electronic device.

FIGS. 21A and 21B show characteristics of a conventional secondarybattery.

FIGS. 22A to 22C are a STEM image and EDX mappings of a positiveelectrode active material in Example 1.

FIGS. 23A to 23C are a STEM image and EDX mappings of a positiveelectrode active material in Example 1.

FIG. 24 is a graph showing the magnesium content in the vicinity of thesurface of a positive electrode active material in Example 1.

FIGS. 25A and 25B are graphs showing cycle characteristics of asecondary battery in Example 1.

FIG. 26 is a graph showing cycle characteristics of a secondary batteryin Example 1.

FIG. 27 is a graph showing cycle characteristics of a secondary batteryin Example 1.

FIGS. 28A and 28B are graphs showing cycle characteristics of asecondary battery in Example 1.

FIGS. 29A and 29B are STEM images of a positive electrode activematerial in Example 2.

FIGS. 30A and 30B are STEM images of a positive electrode activematerial in Example 2.

FIGS. 31A to 31C are graphs showing charge and discharge characteristicsof a secondary battery in Example 2.

FIGS. 32A and 32B are graphs showing cycle characteristics of asecondary battery in Example 2.

FIGS. 33A to 33C are a STEM image and FFT images of a positive electrodeactive material in Example 3.

FIGS. 34A to 34C are a STEM image and FFT images of a positive electrodeactive material in Example 3.

FIGS. 35A to 35C are a STEM image and EDX mappings of a positiveelectrode active material in Example 3.

FIG. 36 is a TEM image of a positive electrode active material inExample 3.

FIGS. 37A1 to 37B2 are a STEM image and EDX mappings of a positiveelectrode active material in Example 3.

FIG. 38 shows ToF-SIMS depth analysis of a positive electrode activematerial in Example 3.

FIG. 39 shows XPS spectra of a positive electrode active material inExample 3.

FIGS. 40A and 40B are graphs showing cycle characteristics of asecondary battery in Example 4.

FIGS. 41A to 41C are graphs showing cycle characteristics of a secondarybattery in Example 4.

FIGS. 42A to 42C are graphs showing cycle characteristics of a secondarybattery in Example 4.

FIGS. 43A and 43B are graphs showing cycle characteristics of asecondary battery in Example 6.

FIGS. 44A and 44B are STEM images of a positive electrode activematerial in Example 6.

FIGS. 45A and 45B are STEM images of a positive electrode activematerial in Example 6.

FIGS. 46A to 46D are a STEM image and FFT images of a positive electrodeactive material in Example 6.

FIG. 47 is a STEM image and an estimation model of a crystal structureof a positive electrode active material in Example 6.

FIGS. 48A1 to 48B2 are a STEM image and EDX mappings of a positiveelectrode active material in Example 6.

FIGS. 49A1 to 49B2 are a STEM image and EDX mappings of a positiveelectrode active material in Example 6.

FIG. 50 is a graph showing EELS analysis results of a positive electrodeactive material in Example 7.

FIG. 51 is a graph showing EELS analysis results of a positive electrodeactive material in Example 7.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. Note that oneembodiment of the present invention is not limited to the descriptionbelow, and it is easily understood by those skilled in the art thatmodes and details of the present invention can be modified in variousways. In addition, the present invention should not be construed asbeing limited to the description in the embodiments given below.

Note that in drawings used in this specification, the sizes,thicknesses, and the like of components such as a positive electrode, anegative electrode, an active material layer, a separator, an exteriorbody are exaggerated for simplicity in some cases. Therefore, the sizesof the components are not limited to the sizes in the drawings andrelative sizes between the components.

Note that in structures of the present invention described in thisspecification and the like, the same portions or portions having similarfunctions are denoted by common reference numerals in differentdrawings, and descriptions thereof are not repeated. Further, the samehatching pattern is applied to portions having similar functions, andthe portions are not especially denoted by reference numerals in somecases.

In this specification and the like, the Miller index is used for theexpression of crystal planes and orientations. In the crystallography, asuperscript bar is placed over a number in the expression using theMiller index; however, in this specification and the like, crystalplanes and orientations are expressed by placing a minus sign (−) at thefront of a number because of expression limitations. Furthermore, anindividual direction which shows an orientation in crystal is denoted by“[ ]”, a set direction which shows all of the equivalent orientations isdenoted by “< >”, an individual direction which shows a crystal plane isdenoted by “( )”, and a set plane having equivalent symmetry is denotedby “{ }”.

In this specification and the like, segregation refers to a phenomenonin which, in a solid made of a plurality of elements (e.g., A, B, andC), a certain element (for example, B) is non-uniformly distributed.

In this specification and the like, a layered rock-salt crystalstructure included in composite oxide containing lithium and atransition metal refers to a crystal structure in which a rock-salt ionarrangement where cations and anions are alternately arranged isincluded and the lithium and the transition metal are regularly arrangedto form a two-dimensional plane, so that lithium can betwo-dimensionally diffused. Note that a defect such as a cation or anionvacancy can exist. In the layered rock-salt crystal structure, strictly,a lattice of a rock-salt crystal is distorted in some cases.

A rock-salt crystal structure refers to a structure in which cations andanions are alternately arranged. Note that a cation or anion vacancy mayexist.

Anions of a layered rock-salt crystal and anions of a rock-salt crystaleach form a cubic closest packed structure (face-centered cubic latticestructure). When a layered rock-salt crystal and a rock-salt crystal arein contact with each other, there is a crystal plane at which directionsof cubic closest packed structures formed of anions are aligned witheach other. Note that a space group of the layered rock-salt crystal isR-3m, which is different from a space group Fm-3m of a rock-saltcrystal; thus, the index of the crystal plane satisfying the aboveconditions in the layered rock-salt crystal is different from that inthe rock-salt crystal. In this specification, in the layered rock-saltcrystal and the rock-salt crystal, a state where the orientations of thecubic closest packed structures formed of anions are aligned with eachother can be referred to as a state where crystal orientations aresubstantially aligned with each other.

For example, when lithium cobalt oxide having a layered rock-salt typecrystal structure and magnesium oxide having a rock-salt type crystalstructure are in contact with each other, the orientation of crystalssubstantially is aligned in the following cases: the (1-1-4) plane oflithium cobalt oxide is in contact with the {001} plane of magnesiumoxide, the (104) plane of lithium cobalt oxide is in contact with the{001} plane of magnesium oxide, the (0-14) plane of lithium cobalt oxideis in contact with {001} plane of the magnesium oxide, the (001) planeof lithium cobalt oxide is in contact with the {111} plane of magnesiumoxide, the (012) plane of lithium cobalt oxide is in contact with the{111} plane of magnesium oxide, and the like.

Whether the crystal orientations in two regions are substantiallyaligned with each other or not can be judged by a transmission electronmicroscope (TEM) image, a scanning transmission electron microscope(STEM) image, a high-angle annular dark field scanning transmissionelectron microscopy (HAADF-STEM) image, an annular bright-field scantransmission electron microscopy (ABF-STEM) image, and the like. X-raydiffraction, electron diffraction, neutron diffraction, and the like canbe used for judging. In the TEM image and the like, alignment of cationsand anions can be observed as repetition of bright lines and dark lines.When the orientations of cubic closest packed structures of the layeredrock-salt crystal and the rock-salt crystal are aligned with each other,a state where an angle between the repetition of bright lines and darklines in the layered rock-salt crystal and the repetition of brightlines and dark lines in the rock-salt crystal is less than or equal to5°, preferably less than or equal to 2.5° is observed. Note that, in theTEM image and the like, a light element such as oxygen or fluorine isnot clearly observed in some cases; however, in such a case, alignmentof orientations can be judged by arrangement of metal elements.

Furthermore, in this specification and the like, a state wherestructures of two-dimensional interfaces have similarity is referred toas “epitaxy”. Crystal growth in which structures of two-dimensionalinterfaces have similarity is referred to as “epitaxial growth”. Inaddition, a state where three-dimensional structures have similarity ororientations are crystallographically the same is referred to as“topotaxy”. Thus, in the case of topotaxy, when part of a cross sectionis observed, orientations of crystals in two regions (e.g., a regionserving as a base and a region formed through growth) are substantiallyaligned with each other.

(Embodiment 1) [Structure of Positive Electrode Active Material]

First, a positive electrode active material 100, which is one embodimentof the present invention, is described with reference to FIGS. 1A to 1C.As shown in FIG. 1A, the positive electrode active material 100 includesa first region 101 and a second region 102. The second region 102 can belocated over the first region 101 or can cover at least part of thefirst region 101.

The first region 101 has composition different from that of the secondregion 102. It is preferable that the second region 102 be a regionwhere segregation of a particular element is observed. Thus, theboundary between the two regions is not clear in some cases. In FIG. 1A,the boundary between the first region 101 and the second region 102 isshown by the dotted lines, and the contrast across the dotted linesmeans a concentration gradient. In FIG. 1B and the following drawings,the boundary between the first region 101 and the second region 102 isshown only by the dotted lines for convenience. The details of the firstregion 101 and the second region 102 are described later.

As illustrated in FIG. 1B, the second region 102 may exist in an innerportion of the positive electrode active material. For example, in thecase where the first region 101 is a polycrystal, segregation of aparticular element may be observed in the grain boundary or in thevicinity thereof to form the second region 102. Furthermore, segregationof a particular element may be observed in a portion which includescrystal defects in the positive electrode active material or in thevicinity thereof to form the second region 102. Note that in thisspecification and the like, crystal defects refer to defects which canbe observed from a TEM image and the like, that is, a structure in whichanother element enters the crystal.

As illustrated in FIG. 1B, the second region 102 does not necessarilycover the entire first region 101.

In other words, the first region 101 exists in an inner portion of thepositive electrode active material 100, and the second region 102 existsin the superficial portion of the positive electrode active material100. In addition, the second region 102 may exist in the inner portionof the positive electrode active material 100.

For example, the first region 101 and the second region 102 can bereferred to as a solid phase A and a solid phase B, respectively.

When the particle size of the positive electrode active material 100 istoo large, problems occur such as difficulty in lithium diffusion andsurface roughness of the active material layer in coating to a currentcollector. In contrast, when the particle size is too small, problemsoccur such as difficulty in supporting of the active material layer incoating to the current collector and over-reaction with an electrolyte.For these reasons, D50 (also referred to as a median diameter) ispreferably 0.1 μm or more and 100 μm or less, further preferably 1 μm ormore and 40 μm or less.

<First Region 101>

The first region 101 contains lithium, a transition metal, and oxygen.In other words, the first region 101 includes composite oxide containinglithium and a transition metal.

As the transition metal contained in the first region 101, a metal thatcan form lithium and layered rock-salt composite oxide is preferablyused. For example, one or a plurality of manganese, cobalt, and nickelcan be used. That is, as the transition metal contained in the firstregion 101, only cobalt may be used, cobalt and manganese may be used,or cobalt, manganese, and nickel may be used. In addition to thetransition metal, the first region 101 may include a metal other thanthe transition metal, such as aluminum.

In other words, the first region 101 can contain a composite oxide oflithium and the transition metal, such as lithium cobalt oxide, lithiumnickel oxide, lithium cobalt oxide in which manganese is substituted forpart of cobalt, lithium nickel-manganese-cobalt oxide and the like, andnickel-cobalt-aluminum oxide and the like.

The first region 101 serves as a region which particularly contributesto a charge and discharge reaction in the positive electrode activematerial 100. To increase capacity of a secondary battery containing thepositive electrode active material 100, the volume of the first region101 is preferably larger than those of the second region 102.

The layered rock-salt crystal structure is preferable for the firstregion 101 because lithium is likely to be diffused two-dimensionally.In addition, when the first region 101 has a layered rock-salt crystalstructure, segregation of a representative element such as magnesium,which is described later, tends to occur unexpectedly. Note that theentire first region 101 does not necessarily have a layered rock-saltcrystal structure. For example, part of the first region 101 may includecrystal defects, may be amorphous, or may have another crystalstructure.

<Second Region 102>

The second region 102 contains magnesium, fluorine, and oxygen. Forexample, the second region 102 may contain magnesium oxide and part ofthe oxygen may be substituted with fluorine.

The second region 102 covers at least part of the first region 101.Magnesium oxide contained in the second region 102 is anelectrochemically stable material that is less likely to deteriorateeven when charge and discharge are repeated; thus, the second region 102is suitable for a coating layer.

When the thickness of the second region 102 is too small, the functionas a coating layer is degraded; however, when the thickness of thesecond region 102 is too large, the capacity is decreased. Thus, thethickness of the second region is preferably greater than or equal to0.5 nm and less than or equal to 50 nm, further preferably greater thanor equal to 0.5 nm and less than or equal to 3 nm.

The second region 102 preferably has a rock-salt crystal structurebecause orientations of crystals are likely to be aligned with those ofthe first region 101, and the second region 102 is likely to function asa stable coating layer. Note that the entire second region 102 does notnecessarily have a rock-salt crystal structure. For example, part of thesecond region 102 may be amorphous or has another crystal structure.

In general, when charge and discharge are repeated, a side reactionoccurs in a positive electrode active material, for example, atransition metal such as manganese or cobalt is dissolved in anelectrolyte solution, oxygen is released, and the crystal structurebecomes unstable, so that the positive electrode active materialdeteriorates. However, the positive electrode active material 100 of oneembodiment of the present invention includes the second region 102 in asuperficial portion; thus, the crystal structure of the composite oxideof lithium and the transition metal contained in the first region 101can be more stable.

The second region 102 contains magnesium, fluorine, and oxygen, andpreferably the same transition metal contained in the first region 101.When the first region 101 and the second region 102 contain the sametransition metal, the valence of the transition metal is preferablydifferent between these two regions. Specifically, it is preferablethat, in the transition metal contained in the first region 101, thenumber of trivalent atoms be larger than the number of atoms exhibitingother valences, and in the transition metal contained in the secondregion 102, the number of divalent atoms be larger than the number ofatoms exhibiting other valences.

When the second region 102 contains many divalent transition metals, itcontains much metal oxide having atomic ratio of transition metal:oxygen=1:1, such as CoO(II), MnO(II), and Ni(II). The metal oxide canform a stable solid solution with magnesium oxide, which is alsodivalent metal oxide. Thus, the second region 102 can be a coveringlayer which is more stable and favorable.

The valence of transition metals can be analyzed by energy lossspectroscopy (EELS), X-ray absorption fine structure (XAFS), X-rayphotoelectron spectroscopy measurement (XPS), electron spin resonance(ESR), Moessbauer spectroscopy, or the like. Among them, EELS ispreferable because of the high spatial resolution. Even if the secondregion 102 is a thin layer with a thickness of about several nanometers,the analysis can be performed.

When the valence of transition metals is analyzed by EELS, the valencecan be determined from the ratio between L₃ and L₂ (L₃/L₂). The largerthe L₃/L₂ is, the higher the proportion of divalent transition metalsis. For example, when transition metals contained in the first region101 and the second region 102 are analyzed by EELS, the L₃/L₂ of thetransition metals contained in the first region 101 and the secondregion 102 are preferably less than 3.8 and more than or equal to 3.8,respectively.

The second region 102 may further contain lithium in addition to theabove-described elements.

It is preferable that the second region 102 also exist in the firstregion 101 as shown in FIG. 1B because the crystal structure ofcomposite oxide containing lithium and a transition metal included inthe first region 101 can be more stable.

In addition, fluorine contained in the second region 102 existspreferably in a bonding state other than MgF₂ and LiF. Specifically,when an XPS analysis is performed on the surface of the positiveelectrode active material 100, a peak position of bonding energy withfluorine and other elements is preferably higher than or equal to 682 eVand lower than 685 eV, further preferably approximately 684.3 eV. Thebonding energy does not correspond to those of MgF₂ and LiF.

In this specification and the like, a peak position of bonding energywith an element in an XPS analysis refers to a value of bonding energyat which the maximum intensity of an energy spectrum is obtained in arange corresponding to bonding energy of the element.

<Boundary Between First Region 101 and Second Region 102>

The difference in composition between the first region 101 and thesecond region 102 can be observed using a TEM image, a STEM image, fastFourier transform (FFT) analysis, energy dispersive X-ray spectrometry(EDX), an analysis in the depth direction by time-of-flight secondaryion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy(XPS), Auger electron spectroscopy, thermal desorption spectroscopy(TDS), or the like. For example, in the cross-sectional TEM image andSTEM image of the positive electrode active material 100, difference ofconstituent elements is observed as difference of brightness; thus,difference of constituent elements of the first region 101 and thesecond region 102 can be observed. Furthermore, it can be observed thatthe first region 101 and the second region 102 contain differentelements from an EDX element distribution image as well. However, clearboundaries between the first region 101 and the second region 102 arenot necessarily observed by the analyses.

In this specification and the like, the range of the second region 102that exists in a superficial portion of the positive electrode activematerial 100 refers to a region from the outermost surface of thepositive electrode active material 100 to a region where a concentrationof a representative element such as magnesium which is detected byanalysis in the depth direction is ⅕ of a peak. As the analysis in thedepth direction, the line analysis of EDX, analysis in the depthdirection using ToF-SIMS, or the like, which is described above, can beused. Furthermore, a concentration peak of magnesium exists preferablyin a region from the surface of the positive electrode active material100 to a depth of 2 nm toward the center, further preferably to a depthof 1 nm, and still further preferably to a depth of 0.5 nm. The depth atwhich the concentration of magnesium becomes ⅕ of the peak, that is therange of the second region 102, is different depending on themanufacturing method. However, in the case of a manufacturing methoddescribed later, the depth is approximately 2 nm to 5 nm from thesurface of the positive electrode active material.

Also in the second region 102 that exists in the first region 101, therange of the second region 102 refers to a region where a concentrationof magnesium which is detected by analysis in the depth direction ishigher than or equal to ⅕ of a peak.

A distribution of fluorine in the positive electrode active material 100preferably overlaps with a distribution of magnesium. Thus, a peak of aconcentration of fluorine preferably exists in a region from the surfaceof the positive electrode active material 100 to a depth of 2 nm towardthe center, further preferably to a depth of 1 nm, and still furtherpreferably to a depth of 0.5 nm.

For these reasons, the second region 102 can be referred to as a densityinclined region in which the concentration of magnesium and fluorine isreduced from the surface of the positive electrode active material 100toward the inner portion.

The concentration of magnesium and fluorine can be analyzed by ToF-SIMS,XPS, Auger electron spectroscopy, TDS, or the like.

Note that the measurement range of the XPS is from the surface of thepositive electrode active material 100 to a region at a depth ofapproximately 5 nm. Thus, the element concentration at a depth ofapproximately 5 nm from the surface can be analyzed quantitatively. Forthis reason, when the thickness of the second region 102 is less than 5nm, the element concentration of the sum of the second region 102 andpart of the first region 101 can be quantitatively analyzed. When thethickness of the second region 102 is 5 nm or more from the surface, theelement concentration of the second region 102 can be quantitativelyanalyzed.

When the surface of the positive electrode active material 100 issubjected to XPS analysis and the total amount of atoms includinglithium, the transition metal contained in the first region 101, oxygen,fluorine, and magnesium is 100 atomic %, the magnesium concentration ispreferably 1 atomic % or more and 16 atomic % or less, and the fluorineconcentration is preferably 0.2 atomic % or more and 4 atomic % or less.Furthermore, the ratio of the concentration of magnesium to fluorine ispreferably Mg:F=y:1 (3≤y≤5) (atomic ratio), and more preferably aboutMg:F=4:1. When the magnesium concentration and the fluorineconcentration are within these ranges, it is possible to obtain thepositive electrode active material 100 exhibiting extremely favorablecycle characteristics when used for a secondary battery.

As described above, the concentrations of magnesium and fluorine aregradually decreased from the surface toward the inner portion; thus, thefirst region 101 may contain the element contained in the second region102, such as magnesium. Similarly, the second region 102 may contain theelement contained in the first region 101. In addition, the first region101 may contain another element such as carbon, sulfur, silicon, sodium,calcium, chlorine, or zirconium. The second region 102 may containanother element such as carbon, sulfur, silicon, sodium, calcium,chlorine, or zirconium.

[Segregation]

The second region 102 can be formed by a sputtering method, a solidphase method, a liquid phase method such as a sol-gel method, or thelike. However, the present inventors found that when a magnesium sourceand a fluorine source are mixed with starting materials and then themixture is heated, the magnesium is segregated to form the second region102. Moreover, they found that the positive electrode active materialincluding the second region 102 formed in this manner has more favorablecharacteristics.

For example, in Example 4 of Patent Document 2 (Patent Application No.2016-076454), a composite oxide containing magnesium is synthesized,then powder of the composite oxide and lithium fluoride are mixed andheated, whereby fluorinated lithiumized surface oxide is formed on thesurface of the composite oxide. According to Example 4, magnesium is notdetected from the surface oxide formed in this method.

However, the present inventors succeeded in segregating magnesium oxideon the superficial portion of the positive electrode active material 100by mixing a magnesium source and a fluorine source with startingmaterials. The present inventors found that the fluorine added to thestarting material exhibited an unexpected effect of segregatingmagnesium.

Since the second region 102 is formed by magnesium segregation,magnesium segregation can occur not only in the superficial portion ofthe positive electrode active material 100 but also in a grain boundaryand the vicinity thereof and crystal defects and the vicinity thereof.The second region 102 formed in a grain boundary and the vicinitythereof and in crystal defects and the vicinity thereof can contributeto further improvement in stability of the crystal structure of thecomposite oxide containing lithium and the transition metal contained inthe first region 101.

The concentration ratio between magnesium and fluorine in the startingmaterials is preferably in a range of Mg:F=1:x (1.5≤x≤4) (atomic ratio),further preferably Mg:F=1:2 (atomic ratio) because the segregation inthe second region 102 effectively occurs.

The concentration ratio between magnesium and fluorine contained in thesecond region 102 formed by segregation is preferably in a range ofMg:F=y:1 (3≤y≤5) (atomic ratio), further preferably Mg:F=about 4:1(atomic ratio), for example.

Since the second region 102 formed by segregation is formed by epitaxialgrowth, orientations of crystals in the first region 101 and the secondregion 102 are partly and substantially aligned with each other in somecases. That is, the first region 101 and the second region 102 becometopotaxy in some cases. When the orientations of crystals in the firstregion 101 and the second region 102 are substantially aligned with eachother, the second region 102 can serve as a more favorable coatinglayer.

<Third Region 103>

It is to be noted that although the example in which the positiveelectrode active material 100 includes the first region 101 and thesecond region 102 has been described so far, one embodiment of thepresent invention is not limited thereto. For example, as illustrated inFIG. 1C, the positive electrode active material 100 may include a thirdregion 103. The third region 103 can be provided, for example, so as tobe in contact with at least a part of the second region 102. The thirdregion 103 may be a coating film containing carbon such as graphenecompounds or may be a coating film containing lithium or an electrolytedecomposition product. When the third region 103 is a coating filmcontaining carbon, it is possible to increase the conductivity betweenthe positive electrode active materials 100 and between the positiveelectrode active material 100 and the current collector. In the casewhere the third region 103 is a coating film having decompositionproducts of lithium or an electrolytic solution, excessive reaction withthe electrolytic solution can be suppressed, and cycle characteristicscan be improved when used for a secondary battery.

Furthermore, a buffer region may be provided between the first region101 and the second region 102. The buffer region preferably containsmetals such as titanium, aluminum, zirconium, and vanadium in additionto lithium, a transition metal, and oxygen, for example. The bufferregion may overlap with the first region 101 and the second region 102.If a positive electrode active material 100 having a buffer region isused, the crystal structure of the first region 101 and the secondregion 102 can be further stabilized and a positive electrode activematerial having excellent cycle characteristics can be obtained, whichis preferable.

[Manufacturing Method]

A manufacturing method of the positive electrode active material 100including the first region 101 and the second region 102 is describedwith reference to FIG. 2 . In this method, the second region 102 isformed by segregation. In the description of this embodiment, atransition metal contained in the first region 101 is cobalt, that is,the first region 101 contains lithium cobalt oxide. Moreover, the secondregion 102 containing magnesium oxide and fluorine is formed bysegregation.

First, starting materials are prepared (S11). Specifically, a lithiumsource, a cobalt source, a magnesium source, and a fluorine source wereindividually weighed. As the lithium source, for example, lithiumcarbonate, lithium fluoride, or lithium hydroxide can be used. As thecobalt source, for example, cobalt oxide, cobalt hydroxide, cobaltoxyhydroxide, cobalt carbonate, cobalt oxalate, cobalt sulfate, or thelike can be used. As the magnesium source, for example, magnesium oxide,magnesium fluoride, or the like can be used. As the fluorine source, forexample, lithium fluoride, magnesium fluoride, or the like can be used.That is, lithium fluoride can be used as both a lithium source and afluorine source. Magnesium fluoride can be used as a magnesium source oras a fluorine source.

In the present embodiment, lithium carbonate (Li₂CO₃) is used as alithium source, cobalt oxide (Co₃O₄) is used as a cobalt source,magnesium oxide (MgO) is used as a magnesium source, lithium fluoride(LiF) is used as a lithium source and a fluorine source.

The atomic ratio of magnesium and fluorine as raw materials ispreferably Mg:F=1:x (1.5≤x≤4) (atomic ratio), more preferably Mg:F=about1:2 (atomic ratio). Therefore, the ratio of magnesium oxide to lithiumfluoride is preferably MgO:LiF=1:x (1.5≤x≤4) (molar ratio), morepreferably MgO:LiF=about 1:2 (atomic ratio).

For example, the molar ratio between the materials can be shown asfollows:

1/2·Li₂CO₃+((1−z)/3)Co₃O₄+z·MgO+2z·LiF (z=0.01)

Next, the weighed starting material is mixed (S12). For example, a ballmill, a bead mill, or the like can be used for the mixing.

Next, the material mixed in S12 is heated (S13). This step can bereferred to as a first heating or baking to distinguish this step fromheating step performed later. The first heating is preferably performedat higher than or equal to 800 ° C. and lower than or equal to 1050 °C., further preferably at higher than or equal to 900° C. and lower thanor equal to 1000° C. The heating time is preferably greater than orequal to 2 hours and less than or equal to 20 hours. The first heatingis preferably performed in a dried atmosphere such as dry air. In thedried atmosphere, for example, the dew point is preferably lower than orequal to −50° C., further preferably lower than or equal to −100° C. Inthis embodiment, the heating is performed at 1000° C. for 10 hours, thetemperature rising rate is 200° C./h, and dry air whose dew point is−109° C. flows at 10 L/min.

By the first heating in S13, a composite oxide containing lithium and atransition metal, that is included in the first region 101, can besynthesized. Also, by the first heating, part of the magnesium andfluorine contained in the starting material is segregated on thesuperficial portion of the composite oxide containing lithium and atransition metal. Note that most of the magnesium and fluorine at thisstage forms a solid solution in the composite oxide containing lithiumand a transition metal.

Next, the material heated in S13 is cooled to room temperature (S14).The cooling time is preferably equal to or longer than the temperaturerising time, for example, 10 hours or longer and 15 hours or shorter.After the cooling, the synthesized material is preferably made to passthrough a sieve. A 53-μm mesh sieve is used in this embodiment.

Note that as the starting materials, particles of the composite oxidecontaining lithium, cobalt, fluorine, and magnesium which aresynthesized in advance may be used. In this case, Step 12 to Step 14 canbe skipped.

Next, the material cooled in S14 is subjected to a second heating (S15).This step can be referred to as a second heating or annealing todistinguish this step from the heating step performed before. Theoptimal conditions for the second heating are changed depending on theparticle size, composition, and the like of the composite oxidecontaining lithium, cobalt, fluorine, and magnesium. However, it ispreferable that the second heating be performed for a holding time at aspecified temperature of 50 hours or shorter, more preferably 2 hours orlonger and 10 hours or shorter. The specified temperature is preferablyhigher than or equal to 500° C. and lower than or equal to 1200° C.,further preferably higher than or equal to 700° C. and lower than orequal to 1000° C., still further preferably about 800° C. The heating isperformed preferably in an oxygen-containing atmosphere. In thisembodiment, the heating is performed at 800° C. for 2 hours, thetemperature rising rate is 200° C./h, and dry air whose dew point is−109° C. flows at 10 L/min.

The second heating in S15 facilitates segregation of the magnesium andfluorine contained in the starting material on the superficial portionof the composite oxide containing lithium and a transition metal.

Finally, the material heated in S15 is cooled to room temperature. Thecooling time is preferably equal to or longer than the temperaturerising time. Then, the cooled material is collected (S16), and thepositive electrode active material 100 including the first region 101and the second region 102 can be obtained.

The use of the positive electrode active material described in thepresent embodiment can provide a secondary battery with high capacityand excellent cycle characteristics. This embodiment can be implementedin appropriate combination with any of the other embodiments.

(Embodiment 2)

In this embodiment, examples of materials which can be used for asecondary battery containing the positive electrode active material 100described in the above embodiment are described. In this embodiment, asecondary battery in which a positive electrode, a negative electrode,and an electrolyte solution are wrapped in an exterior body is describedas an example.

[Positive Electrode]

The positive electrode includes a positive electrode active materiallayer and a positive electrode current collector.

<Positive Electrode Active Material Layer>

The positive electrode active material layer contains a positiveelectrode active material. The positive electrode active material layermay contain a conductive additive and a binder.

As the positive electrode active material, the positive electrode activematerial 100 described in the above embodiment can be used. When theabove-described positive electrode active material 100 is used, asecondary battery with high capacity and excellent cycle characteristicscan be obtained.

Examples of the conductive additive include a carbon material, a metalmaterial, and a conductive ceramic material. Alternatively, a fibermaterial may be used as the conductive additive. The content of theconductive additive with respect to the total amount of the activematerial layer is preferably greater than or equal to 1 wt % and lessthan or equal to 10 wt %, more preferably greater than or equal to 1 wt% and less than or equal to 5 wt %.

A network for electric conduction can be formed in the electrode by theconductive additive. The conductive additive also allows maintaining ofa path for electric conduction between the positive electrode activematerial particles. The addition of the conductive additive to theactive material layer increases the electric conductivity of the activematerial layer.

Examples of the conductive additive include natural graphite, artificialgraphite such as mesocarbon microbeads, and carbon fiber. Examples ofcarbon fiber include mesophase pitch-based carbon fiber, isotropicpitch-based carbon fiber, carbon nanofiber, and carbon nanotube. Carbonnanotube can be formed by, for example, a vapor deposition method. Otherexamples of the conductive additive include carbon materials such ascarbon black (e.g., acetylene black (AB)), graphite (black lead)particles, graphene, and fullerene. Alternatively, metal powder or metalfibers of copper, nickel, aluminum, silver, gold, or the like, aconductive ceramic material, or the like can be used.

Alternatively, a graphene compound may be used as the conductiveadditive.

A graphene compound has excellent electrical characteristics of highconductivity and excellent physical properties of high flexibility andhigh mechanical strength. Furthermore, a graphene compound has a planarshape. A graphene compound enables low-resistance surface contact.Furthermore, a graphene compound has extremely high conductivity evenwith a small thickness in some cases and thus allows a conductive pathto be formed in an active material layer efficiently even with a smallamount. For this reason, it is preferable to use a graphene compound asthe conductive additive because the area where the active material andthe conductive additive are in contact with each other can be increased.Here, it is particularly preferable to use, for example, graphene,multilayer graphene, or reduced graphene oxide (hereinafter “RGO”) as agraphene compound. Note that RGO refers to a compound obtained byreducing graphene oxide (GO), for example.

In the case where an active material with a small particle diameter(e.g., 1 μm or less) is used, the specific surface area of the activematerial is large and thus more conductive paths for the active materialparticles are needed. Thus, the amount of conductive additive tends toincrease and the supported amount of active material tends to decreaserelatively. When the supported amount of active material decreases, thecapacity of the secondary battery also decreases. In such a case, agraphene compound that can efficiently form a conductive path even in asmall amount is particularly preferably used as the conductive additivebecause the supported amount of active material does not decrease.

A cross-sectional structure example of an active material layer 200containing a graphene compound as a conductive additive is describedbelow.

FIG. 3A shows a longitudinal cross-sectional view of the active materiallayer 200. The active material layer 200 includes positive electrodeactive material particles 100, a graphene compound 201 serving as aconductive additive, and a binder (not illustrated). Here, graphene ormultilayer graphene may be used as the graphene compound 201, forexample. The graphene compound 201 preferably has a sheet-like shape.The graphene compound 201 may have a sheet-like shape formed of aplurality of sheets of multilayer graphene and/or a plurality of sheetsof graphene that partly overlap with each other.

The longitudinal cross section of the active material layer 200 in FIG.3A shows substantially uniform dispersion of the sheet-like graphenecompounds 201 in the active material layer 200. The graphene compounds201 are schematically shown by thick lines in FIG. 3A but are actuallythin films each having a thickness corresponding to the thickness of asingle layer or a multi-layer of carbon molecules. The plurality ofgraphene compounds 201 are formed in such a way as to partly coat oradhere to the surfaces of the plurality of positive electrode activematerial particles 100, so that the graphene compounds 201 make surfacecontact with the positive electrode active material particles 100.

Here, the plurality of graphene compounds are bonded to each other toform a net-like graphene compound sheet (hereinafter referred to as agraphene compound net or a graphene net). The graphene net covering theactive material can function as a binder for bonding active materials.The amount of a binder can thus be reduced, or the binder does not haveto be used. This can increase the proportion of the active material inthe electrode volume or weight. That is to say, the capacity of thestorage device can be increased.

Here, it is preferable to perform reduction after a layer to be theactive material layer 200 is formed in such a manner that graphene oxideis used as the graphene compound 201 and mixed with an active material.When graphene oxide with extremely high dispersibility in a polarsolvent is used for the formation of the graphene compounds 201, thegraphene compounds 201 can be substantially uniformly dispersed in theactive material layer 200. The solvent is removed by volatilization froma dispersion medium in which graphene oxide is uniformly dispersed, andthe graphene oxide is reduced; hence, the graphene compounds 201remaining in the active material layer 200 partly overlap with eachother and are dispersed such that surface contact is made, therebyforming a three-dimensional conduction path. Note that graphene oxidecan be reduced either by heat treatment or with the use of a reducingagent, for example.

Unlike a conductive additive in the form of particles, such as acetyleneblack, which makes point contact with an active material, the graphenecompound 201 is capable of making low-resistance surface contact;accordingly, the electrical conduction between the positive electrodeactive material particles 100 and the graphene compounds 201 can beimproved with a smaller amount of the graphene compound 201 than that ofa normal conductive additive. This increases the proportion of thepositive electrode active material 100 in the active material layer 200,resulting in increased discharge capacity of the storage device.

As the binder, a rubber material such as styrene-butadiene rubber (SBR),styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber,butadiene rubber, or ethylene-propylene-diene copolymer can be used, forexample. Alternatively, fluororubber can be used as the binder.

For the binder, for example, water-soluble polymers are preferably used.As the water-soluble polymers, a polysaccharide and the like can beused. As the polysaccharide, a cellulose derivative such ascarboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose,hydroxypropyl cellulose, diacetyl cellulose, or regenerated cellulose,starch, or the like can be used. It is more preferred that suchwater-soluble polymers be used in combination with any of the aboverubber materials.

Alternatively, as the binder, a material such as polystyrene,poly(methyl acrylate), poly(methyl methacrylate) (PMMA), sodiumpolyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO),polypropylene oxide, polyimide, polyvinyl chloride,polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene,polyethylene terephthalate, nylon, polyvinylidene fluoride (PVdF),polyacrylonitrile (PAN), ethylene-propylene-diene polymer, polyvinylacetate, or nitrocellulose is preferably used.

A plurality of the above materials may be used in combination for thebinder.

For example, a material having a significant viscosity modifying effectand another material may be used in combination. For example, a rubbermaterial or the like has high adhesion or high elasticity but may havedifficulty in viscosity modification when mixed in a solvent. In such acase, a rubber material or the like is preferably mixed with a materialhaving a significant viscosity modifying effect, for example. As amaterial having a significant viscosity modifying effect, for example, awater-soluble polymer is preferably used. An example of a water-solublepolymer having an especially significant viscosity modifying effect isthe above-mentioned polysaccharide; for example, a cellulose derivativesuch as carboxymethyl cellulose (CMC), methyl cellulose, ethylcellulose, hydroxypropyl cellulose, diacetyl cellulose, or regeneratedcellulose, or starch can be used.

Note that a cellulose derivative such as carboxymethyl cellulose obtainsa higher solubility when converted into a salt such as a sodium salt oran ammonium salt of carboxymethyl cellulose, and accordingly, easilyexerts an effect as a viscosity modifier. The high solubility can alsoincrease the dispersibility of an active material and other componentsin the formation of slurry for an electrode. In this specification,cellulose and a cellulose derivative used as a binder of an electrodeinclude salts thereof.

The water-soluble polymers stabilize viscosity by being dissolved inwater and allow stable dispersion of the active material and anothermaterial combined as a binder such as styrene-butadiene rubber in anaqueous solution. Furthermore, a water-soluble polymer is expected to beeasily and stably adsorbed to an active material surface because it hasa functional group. Many cellulose derivatives such as carboxymethylcellulose have functional groups such as a hydroxyl group and a carboxylgroup. Because of functional groups, polymers are expected to interactwith each other and cover an active material surface in a large area.

In the case where the binder covering or being in contact with theactive material surface forms a film, the film is expected to serve as apassivation film to suppress the decomposition of the electrolytesolution. Here, the passivation film refers to a film without electricconductivity or a film with extremely low electric conductivity, and caninhibit the decomposition of an electrolyte solution at a potential atwhich a battery reaction occurs in the case where the passivation filmis formed on the active material surface, for example. It is preferredthat the passivation film can conduct lithium ions while suppressingelectric conduction.

<Positive Electrode Current Collector>

The positive electrode current collector can be formed using a materialthat has high conductivity, such as a metal like stainless steel, gold,platinum, aluminum, or titanium, or an alloy thereof. It is preferredthat a material used for the positive electrode current collector notdissolve at the potential of the positive electrode. Alternatively, thepositive electrode current collector can be formed using an aluminumalloy to which an element that improves heat resistance, such assilicon, titanium, neodymium, scandium, or molybdenum, is added. Stillalternatively, a metal element that forms silicide by reacting withsilicon can be used. Examples of the metal element that forms silicideby reacting with silicon include zirconium, titanium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel.The current collector can have any of various shapes including afoil-like shape, a plate-like shape (sheet-like shape), a net-likeshape, a punching-metal shape, and an expanded-metal shape. The currentcollector preferably has a thickness of 5 μm to 30 μm.

[Negative Electrode]

The negative electrode includes a negative electrode active materiallayer and a negative electrode current collector. The negative electrodeactive material layer may contain a conductive additive and a binder.

<Negative Electrode Active Material>

As a negative electrode active material, for example, an alloy-basedmaterial or a carbon-based material can be used.

For the negative electrode active material, an element which enablescharge-discharge reactions by an alloying reaction and a dealloyingreaction with lithium can be used. For example, a material containing atleast one of silicon, tin, gallium, aluminum, germanium, lead, antimony,bismuth, silver, zinc, cadmium, indium, and the like can be used. Suchelements have higher capacity than carbon. In particular, silicon has asignificantly high theoretical capacity of 4200 mAh/g. For this reason,silicon is preferably used as the negative electrode active material.Alternatively, a compound containing any of the above elements may beused. Examples of the compound include SiO, Mg₂Si, Mg₂Ge, SnO, SnO_(2,)Mg₂Sn, SnS₂, V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb,CeSb₃, LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, and SbSn. Here, an element thatenables charge-discharge reactions by an alloying reaction and adealloying reaction with lithium, a compound containing the element, andthe like may be referred to as an alloy-based material.

In this specification and the like, SiO refers, for example, to siliconmonoxide. SiO can alternatively be expressed as SiO_(x). Here, xpreferably has an approximate value of 1. For example, x is preferably0.2 or more and 1.5 or less, more preferably 0.3 or more and 1.2 orless.

As the carbon-based material, graphite, graphitizing carbon (softcarbon), non-graphitizing carbon (hard carbon), a carbon nanotube,graphene, carbon black, and the like can be used.

Examples of graphite include artificial graphite and natural graphite.Examples of artificial graphite include meso-carbon microbeads (MCMB),coke-based artificial graphite, and pitch-based artificial graphite. Asartificial graphite, spherical graphite having a spherical shape can beused. For example, MCMB is preferably used because it may have aspherical shape. Moreover, MCMB may preferably be used because it canrelatively easily have a small surface area. Examples of naturalgraphite include flake graphite and spherical natural graphite.

Graphite has a low potential substantially equal to that of a lithiummetal (higher than or equal to 0.05 V and lower than or equal to 0.3 Vvs. Li/Li⁺) when lithium ions are intercalated into the graphite (whilea lithium-graphite intercalation compound is formed). For this reason, alithium-ion secondary battery can have a high operating voltage. Inaddition, graphite is preferred because of its advantages such as arelatively high capacity per unit volume, relatively small volumeexpansion, low cost, and higher level of safety than that of a lithiummetal.

Alternatively, for the negative electrode active material, an oxide suchas titanium dioxide (TiO₂), lithium titanium oxide (Li₄Ti₅O₁₂),lithium-graphite intercalation compound (Li_(x)C₆), niobium pentoxide(Nb₂O₅), tungsten oxide (WO₂), or molybdenum oxide (MoO₂) can be used.

Still alternatively, for the negative electrode active material,Li_(3−x)M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g and 1890 mAh/cm³).

A nitride containing lithium and a transition metal is preferably used,in which case lithium ions are contained in the negative electrodeactive material and thus the negative electrode active material can beused in combination with a material for a positive electrode activematerial which does not contain lithium ions, such as V₂O₅ or Cr₃O₈. Inthe case of using a material containing lithium ions as a positiveelectrode active material, the nitride containing lithium and atransition metal can be used for the negative electrode active materialby extracting the lithium ions contained in the positive electrodeactive material in advance.

Alternatively, a material which causes a conversion reaction can be usedfor the negative electrode active material; for example, a transitionmetal oxide which does not form an alloy with lithium, such as cobaltoxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may be used.Other examples of the material which causes a conversion reactioninclude oxides such as Fe₂O₃, CuO, Cu₂O, RuO₂, and Cr₂O₃, sulfides suchas CoS_(0.89), NiS, and CuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄,phosphides such as NiP₂, FeP₂, and CoP₃, and fluorides such as FeF₃ andBiF₃.

For the conductive additive and the binder that can be included in thenegative electrode active material layer, materials similar to those ofthe conductive additive and the binder that can be included in thepositive electrode active material layer can be used.

<Negative Electrode Current Collector>

For the negative electrode current collector, a material similar to thatof the positive electrode current collector can be used. Note that amaterial which is not alloyed with a carrier ion such as lithium ispreferably used for the negative electrode current collector.

[Electrolyte Solution]

The electrolyte solution contains a solvent and an electrolyte. As asolvent of the electrolyte solution, an aprotic organic solvent ispreferably used. For example, one of ethylene carbonate (EC), propylenecarbonate (PC), butylene carbonate, chloroethylene carbonate, vinylenecarbonate, y-butyrolactone, y-valerolactone, dimethyl carbonate (DMC),diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate,methyl acetate, ethyl acetate, methyl propionate, ethyl propionate,propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane,dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyldiglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, andsultone can be used, or two or more of these solvents can be used in anappropriate combination in an appropriate ratio.

When a gelled high-molecular material is used as the solvent of theelectrolytic solution, safety against liquid leakage and the like isimproved. Furthermore, a secondary battery can be thinner and morelightweight. Typical examples of gelled high-molecular materials includea silicone gel, an acrylic gel, an acrylonitrile gel, a polyethyleneoxide-based gel, a polypropylene oxide-based gel, a gel of afluorine-based polymer, and the like.

Alternatively, when one or more kinds of ionic liquids (room temperaturemolten salts) which have features of non-flammability and non-volatilityis used as a solvent of the electrolyte solution, a secondary batterycan be prevented from exploding or catching fire even when the secondarybattery internally shorts out or the internal temperature increasesowing to overcharging or the like. An ionic liquid contains a cation andan anion. The ionic liquid contains an organic cation and an anion.Examples of the organic cation used for the electrolyte solution includealiphatic onium cations such as a quaternary ammonium cation, a tertiarysulfonium cation, and a quaternary phosphonium cation, and aromaticcations such as an imidazolium cation and a pyridinium cation. Examplesof the anion used for the electrolyte solution include a monovalentamide-based anion, a monovalent methide-based anion, a fluorosulfonateanion, a perfluoroalkylsulfonate anion, a tetrafluoroborate anion, aperfluoroalkylborate anion, a hexafluorophosphate anion, and aperfluoroalkylphosphate anion.

As an electrolyte dissolved in the above-described solvent, one oflithium salts such as LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiAlCl₄, LiSCN,LiBr, LiI, Li₂SO₄, Li₂B₁₀Cl₁₀, Li₂B₁₂Cl₁₂, LiCF₃SO₃, LiC₄F₉SO₃,LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃, LiN(CF₃SO₂)₂, LiN(C₄F₉SO₂) (CF₃SO₂), andLiN(C₂F₅SO₂)₂ can be used, or two or more of these lithium salts can beused in an appropriate combination in an appropriate ratio.

The electrolyte solution used for a storage device is preferably highlypurified and contains a small amount of dust particles and elementsother than the constituent elements of the electrolyte solution(hereinafter also simply referred to as impurities). Specifically, theweight ratio of impurities to the electrolyte solution is less than orequal to 1%, preferably less than or equal to 0.1%, and furtherpreferably less than or equal to 0.01%.

Furthermore, an additive agent such as vinylene carbonate, propanesultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC),LiBOB, or a dinitrile compound such as succinonitrile or adiponitrilemay be added to the electrolyte solution. The concentration of amaterial to be added with respect to the whole solvent is, for example,higher than or equal to 0.1 wt % and lower than or equal to 5 wt %.

Alternatively, a gelled electrolyte obtained in such a manner that apolymer is swelled with an electrolyte solution may be used. When apolymer gel electrolyte is used, safety against liquid leakage and thelike is improved. Furthermore, a secondary battery can be thinner andmore lightweight.

As a polymer that undergoes gelation, a silicone gel, an acrylic gel, anacrylonitrile gel, a polyethylene oxide-based gel, a polypropyleneoxide-based gel, a fluorine-based polymer gel, or the like can be used.

Examples of the polymer include a polymer having a polyalkylene oxidestructure, such as polyethylene oxide (PEO); PVDF; polyacrylonitrile;and a copolymer containing any of them. For example, PVDF-HFP, which isa copolymer of PVDF and hexafluoropropylene (HFP) can be used. Theformed polymer may be porous.

Instead of the electrolyte solution, a solid electrolyte including aninorganic material such as a sulfide-based inorganic material or anoxide-based inorganic material, or a solid electrolyte including ahigh-molecular material such as a polyethylene oxide (PEO)-basedhigh-molecular material may alternatively be used. When the solidelectrolyte is used, a separator and a spacer are not necessary.Furthermore, since the battery can be entirely solidified, there is nopossibility of liquid leakage to increase the safety of the batterydramatically.

[Separator]

The secondary battery preferably includes a separator. As the separator,for example, paper; nonwoven fabric; glass fiber; ceramics; or syntheticfiber using nylon (polyamide), vinylon (polyvinyl alcohol-based fiber),polyester, acrylic, polyolefin, or polyurethane can be used. Theseparator is preferably formed to have an envelope-like shape to wrapone of the positive electrode and the negative electrode.

The separator may have a multilayer structure. For example, an organicmaterial film such as polypropylene or polyethylene can be coated with aceramic-based material, a fluorine-based material, a polyamide-basedmaterial, a mixture thereof, or the like. Examples of the ceramic-basedmaterial include aluminum oxide particles and silicon oxide particles.Examples of the fluorine-based material include PVDF and apolytetrafluoroethylene. Examples of the polyamide-based materialinclude nylon and aramid (meta-based aramid and para-based aramid).

Deterioration of the separator in charging and discharging at highvoltage can be suppressed and thus the reliability of the secondarybattery can be improved because oxidation resistance is improved whenthe separator is coated with the ceramic-based material. In addition,when the separator is coated with the fluorine-based material, theseparator is easily brought into close contact with an electrode,resulting in high output characteristics. When the separator is coatedwith the polyamide-based material, in particular, aramid, the safety ofthe secondary battery is improved because heat resistance is improved.

For example, both surfaces of a polypropylene film may be coated with amixed material of aluminum oxide and aramid. Alternatively, a surface ofthe polypropylene film in contact with the positive electrode may becoated with the mixed material of aluminum oxide and aramid, and asurface of the polypropylene film in contact with the negative electrodemay be coated with the fluorine-based material.

With the use of a separator having a multilayer structure, the capacityof the secondary battery per volume can be increased because the safetyof the secondary battery can be maintained even when the total thicknessof the separator is small.

(Embodiment 3)

In this embodiment, examples of a shape of a secondary batterycontaining the positive electrode active material 100 described in theabove embodiment are described. For the materials used for the secondarybattery described in this embodiment, the description of the aboveembodiment can be referred to.

[Coin-Type Secondary Battery]

First, an example of a coin-type secondary battery is described. FIG. 4Ais an external view of a coin-type (single-layer flat type) secondarybattery, and FIG. 4B is a cross-sectional view thereof.

In a coin-type secondary battery 300, a positive electrode can 301doubling as a positive electrode terminal and a negative electrode can302 doubling as a negative electrode terminal are insulated from eachother and sealed by a gasket 303 made of polypropylene or the like. Apositive electrode 304 includes a positive electrode current collector305 and a positive electrode active material layer 306 provided incontact with the positive electrode current collector 305. A negativeelectrode 307 includes a negative electrode current collector 308 and anegative electrode active material layer 309 provided in contact withthe negative electrode current collector 308.

Note that only one surface of each of the positive electrode 304 and thenegative electrode 307 used for the coin-type secondary battery 300 isprovided with an active material layer.

For the positive electrode can 301 and the negative electrode can 302, ametal having a corrosion-resistant property to an electrolyte solution,such as nickel, aluminum, or titanium, an alloy of such a metal, or analloy of such a metal and another metal (e.g., stainless steel) can beused. Alternatively, the positive electrode can 301 and the negativeelectrode can 302 are preferably covered with nickel, aluminum, or thelike in order to prevent corrosion due to the electrolyte solution. Thepositive electrode can 301 and the negative electrode can 302 areelectrically connected to the positive electrode 304 and the negativeelectrode 307, respectively.

The negative electrode 307, the positive electrode 304, and theseparator 310 are immersed in the electrolyte solution. Then, asillustrated in FIG. 4B, the positive electrode 304, the separator 310,the negative electrode 307, and the negative electrode can 302 arestacked in this order with the positive electrode can 301 positioned atthe bottom, and the positive electrode can 301 and the negativeelectrode can 302 are subjected to pressure bonding with the gasket 303located therebetween. In such a manner, the coin-type secondary battery300 can be manufactured.

When the positive electrode active material described in the aboveembodiment is used in the positive electrode 304, the coin-typesecondary battery 300 with high capacity and excellent cyclecharacteristics can be obtained.

[Cylindrical Secondary Battery]

Next, an example of a cylindrical secondary battery will be describedwith reference to FIGS. 5A to 5D. A cylindrical secondary battery 600includes, as illustrated in FIG. 5A, a positive electrode cap (batterylid) 601 on the top surface and a battery can (outer can) 602 on theside and bottom surfaces. The positive electrode cap and the battery can(outer can) 602 are insulated from each other by a gasket (insulatingpacking) 610.

FIG. 5B is a schematic cross-sectional view of the cylindrical secondarybattery. Inside the battery can 602 having a hollow cylindrical shape, abattery element in which a strip-like positive electrode 604 and astrip-like negative electrode 606 are wound with a strip-like separator605 located therebetween is provided. Although not illustrated, thebattery element is wound around a center pin. One end of the battery can602 is close and the other end thereof is open. For the battery can 602,a metal having a corrosion-resistant property to an electrolytesolution, such as nickel, aluminum, or titanium, an alloy of such ametal, or an alloy of such a metal and another metal (e.g., stainlesssteel) can be used. Alternatively, the battery can 602 is preferablycovered with nickel, aluminum, or the like in order to prevent corrosiondue to the electrolyte solution. Inside the battery can 602, the batteryelement in which the positive electrode, the negative electrode, and theseparator are wound is provided between a pair of insulating plates 608and 609 that face each other. Furthermore, a nonaqueous electrolytesolution (not illustrated) is injected inside the battery can 602provided with the battery element. As the nonaqueous electrolytesolution, a nonaqueous electrolyte solution that is similar to that ofthe coin-type secondary battery can be used.

Since the positive electrode and the negative electrode of thecylindrical secondary battery are wound, active materials are preferablyformed on both sides of the current collectors. A positive electrodeterminal (positive electrode current collecting lead) 603 is connectedto the positive electrode 604, and a negative electrode terminal(negative electrode current collecting lead) 607 is connected to thenegative electrode 606. Both the positive electrode terminal 603 and thenegative electrode terminal 607 can be formed using a metal materialsuch as aluminum. The positive electrode terminal 603 and the negativeelectrode terminal 607 are resistance-welded to a safety valve mechanism612 and the bottom of the battery can 602, respectively. The safetyvalve mechanism 612 is electrically connected to the positive electrodecap 601 through a positive temperature coefficient (PTC) element 611.The safety valve mechanism 612 cuts off electrical connection betweenthe positive electrode cap 601 and the positive electrode 604 when theinternal pressure of the battery exceeds a predetermined thresholdvalue. The PTC element 611, which serves as a thermally sensitiveresistor whose resistance increases as temperature rises, limits theamount of current by increasing the resistance, in order to preventabnormal heat generation. Barium titanate (BaTiO₃)-based semiconductorceramic can be used for the PTC element.

Alternatively, as illustrated in FIG. 5C, a plurality of secondarybatteries 600 may be sandwiched between a conductive plate 613 and aconductive plate 614 to form a module 615. The plurality of secondarybatteries 600 may be connected parallel to each other, connected inseries, or connected in series after being connected parallel to eachother. With the module 615 including the plurality of secondarybatteries 600, large electric power can be extracted.

FIG. 5D is a top view of the module 615. The conductive plate 613 isshown by a dotted line for clarity of the drawing. As illustrated inFIG. 5D, the module 615 may include a wiring 616 which electricallyconnects the plurality of secondary batteries 600 to each other. It ispossible to provide the conductive plate 613 over the wiring 616 tooverlap with each other. In addition, a temperature control device 617may be provided between the plurality of secondary batteries 600. Whenthe secondary batteries 600 are overheated, the temperature controldevice 617 can cool them, and when the secondary batteries 600 arecooled too much, the temperature control device 617 can heat them. Thus,the performance of the module 615 is not easily influenced by theoutside air temperature.

When the positive electrode active material described in the aboveembodiment is used in the positive electrode 604, the cylindricalsecondary battery 600 with high capacity and excellent cyclecharacteristics can be obtained.

[Structural Examples of Secondary Battery]

Other structural examples of secondary batteries will be described withreference to FIGS. 6A and 6B, FIGS. 7A-1, 7A-2, 7B-1, and 7B-2 , FIGS.8A and 8B, and FIG. 9 .

FIGS. 6A and 6B are external views of a battery pack. The battery packincludes a circuit board 900 and a secondary battery 913. The secondarybattery 913 includes a terminal 951 and a terminal 952 and is covered bya label 910. The battery pack may include an antenna 914.

The circuit board 900 is fixed by a sealant 915. The circuit board 900includes a circuit 912. The terminal 911 is electrically connected tothe terminals 951 and 952 of the secondary battery 913 with the circuitboard 900 therebetween. The terminal 911 is electrically connected tothe antenna 914 and the circuit 912 with the circuit board 900therebetween. Note that a plurality of terminals 911 serving as acontrol signal input terminal, a power supply terminal, and the like maybe provided.

For example, the circuit 912 serves as a protection circuit forprotecting the secondary battery 913 from overcharge, overdischarge, andovercurrent. The circuit 912 may be provided on the rear surface of thecircuit board 900. Note that the shape of the antenna 914 is not limitedto a coil shape and may be a linear shape or a plate shape. Further, aplanar antenna, an aperture antenna, a traveling-wave antenna, an EHantenna, a magnetic-field antenna, or a dielectric antenna may be used.The antenna 914 has a function of communicating data with an externaldevice, for example. As a system for communication using the antenna 914between the battery pack and another device, a response method that canbe used between the storage battery and another device, such as NFC, canbe employed.

The battery pack includes a layer 916 between the secondary battery 913and the antenna 914. The layer 916 has a function of blocking anelectromagnetic field from the secondary battery 913, for example. Asthe layer 916, for example, a magnetic body can be used.

Note that the structure of the battery pack is not limited to that shownin FIGS. 6A and 6B.

For example, as shown in FIGS. 7A-1 and 7A-2 , two opposite surfaces ofthe secondary battery 913 in FIGS. 6A and 6B may be provided withrespective antennas. FIG. 7A-1 is an external view showing one side ofthe opposite surfaces, and FIG. 7A-2 is an external view showing theother side of the opposite surfaces. For portions similar to those inFIGS. 6A and 6B, a description of the battery pack illustrated in FIGS.6A and 6B can be referred to as appropriate.

As illustrated in FIG. 7A-1 , the antenna 914 is provided on one of theopposite surfaces of the secondary battery 913 with the layer 916located therebetween, and as illustrated in FIG. 7A-2 , an antenna 918is provided on the other of the opposite surfaces of the secondarybattery 913 with a layer 917 located therebetween. The layer 917 has afunction of blocking an electromagnetic field from the secondary battery913, for example. As the layer 917, for example, a magnetic body can beused.

With the above structure, the battery pack can have two antennas andboth of the antennas 914 and 918 can be increased in size.

An antenna with a shape that can be applied to the antenna 914 can beused as the antenna 918. The antenna 918 may be a flat-plate conductor.The flat-plate conductor can serve as one of conductors for electricfield coupling. That is, the antenna 914 can serve as one of twoconductors of a capacitor. Thus, electric power can be transmitted andreceived not only by an electromagnetic field or a magnetic field butalso by an electric field.

Alternatively, as illustrated in FIG. 7B-1 , the battery pack in FIGS.6A and 6B may be provided with a display device 920. The display device920 is electrically connected to the terminal 911. For portions similarto those in FIGS. 6A and 6B, a description of the battery packillustrated in FIGS. 6A and 6B can be referred to as appropriate.

The display device 920 can display, for example, an image showingwhether charge is being carried out, an image showing the amount ofstored power, or the like. As the display device 920, electronic paper,a liquid crystal display device, an electroluminescent (EL) displaydevice, or the like can be used. For example, the use of electronicpaper can reduce power consumption of the display device 920.

Alternatively, as illustrated in FIG. 7B-2 , the secondary battery 913illustrated in FIGS. 6A and 6B may be provided with a sensor 921. Thesensor 921 is electrically connected to the terminal 911 via a terminal922 and the circuit board 900. For portions similar to those in FIGS. 6Aand 6B, a description of the storage device illustrated in FIGS. 6A and6B can be referred to as appropriate.

The sensor 921 has a function of measuring, for example, displacement,position, speed, acceleration, angular velocity, rotational frequency,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, electric current, voltage,electric power, radiation, flow rate, humidity, gradient, oscillation,odor, or infrared rays. With the sensor 921, for example, data on anenvironment (e.g., temperature) where the storage device is placed canbe determined and stored in a memory inside the circuit 912.

Furthermore, structural examples of the secondary battery 913 will bedescribed with reference to FIGS. 8A and 8B and FIG. 9 .

The secondary battery 913 illustrated in FIG. 8A includes a wound body950 provided with the terminals 951 and 952 inside a housing 930. Thewound body 950 is soaked in an electrolyte solution inside the housing930. The terminal 952 is in contact with the housing 930. An insulatoror the like inhibits contact between the terminal 951 and the housing930. Note that in FIG. 8A, the housing 930 divided into two pieces isillustrated for convenience; however, in the actual structure, the woundbody 950 is covered with the housing 930 and the terminals 951 and 952extend to the outside of the housing 930. For the housing 930, a metalmaterial (such as aluminum) or a resin material can be used.

Note that as illustrated in FIG. 8B, the housing 930 in FIG. 8A may beformed using a plurality of materials. For example, in the secondarybattery 913 in FIG. 8B, a housing 930 a and a housing 930 b are bondedto each other, and the wound body 950 is provided in a region surroundedby the housing 930 a and the housing 930 b.

For the housing 930 a, an insulating material such as an organic resincan be used. In particular, when a material such as an organic resin isused for the side on which an antenna is formed, blocking of an electricfield from the secondary battery 913 can be inhibited. When an electricfield is not significantly blocked by the housing 930 a, an antenna suchas the antennas 914 and 918 may be provided inside the housing 930 a.For the housing 930 b, a metal material can be used, for example.

FIG. 9 illustrates the structure of the wound body 950. The wound body950 includes a negative electrode 931, a positive electrode 932, andseparators 933. The wound body 950 is obtained by winding a sheet of astack in which the negative electrode 931 overlaps with the positiveelectrode 932 with the separator 933 provided therebetween. Note that aplurality of stacks each including the negative electrode 931, thepositive electrode 932, and the separator 933 may be stacked.

The negative electrode 931 is connected to the terminal 911 in FIGS. 6Aand 6B via one of the terminals 951 and 952. The positive electrode 932is connected to the terminal 911 in FIGS. 6A and 6B via the other of theterminals 951 and 952.

When the positive electrode active material described in the aboveembodiment is used in the positive electrode 932, the secondary battery913 with high capacity and excellent cycle characteristics can beobtained.

[Laminated Secondary Battery]

Next, an example of a laminated secondary battery will be described withreference to FIGS. 10A to 10C, FIG. 11A and 11B, FIG. 12 , FIG. 13 ,FIGS. 14A to 14C, FIGS. 15A, 15B1, 15B2, 15C, and 15D, and FIGS. 16A and16B. When the laminated secondary battery has flexibility and is used inan electronic device at least part of which is flexible, the secondarybattery can be bent as the electronic device is bent.

A laminated secondary battery 980 is described with reference to FIGS.10A to 10C. The laminated secondary battery 980 includes a wound body993 illustrated in FIG. 10A. The wound body 993 includes a negativeelectrode 994, a positive electrode 995, and a separator 996. The woundbody 993 is, like the wound body 950 illustrated in FIG. 9 , obtained bywinding a sheet of a stack in which the negative electrode 994 overlapswith the positive electrode 995 with the separator 996 therebetween.

Note that the number of stacks each including the negative electrode994, the positive electrode 995, and the separator 996 may be determinedas appropriate depending on capacity and an element volume which arerequired. The negative electrode 994 is connected to a negativeelectrode current collector (not illustrated) via one of a leadelectrode 997 and a lead electrode 998. The positive electrode 995 isconnected to a positive electrode current collector (not illustrated)via the other of the lead electrode 997 and the lead electrode 998.

As illustrated in FIG. 10B, the wound body 993 is packed in a spaceformed by bonding a film 981 and a film 982 having a depressed portionthat serve as exterior bodies by thermocompression bonding or the like,whereby the secondary battery 980 can be formed as illustrated in FIG.10C. The wound body 993 includes the lead electrode 997 and the leadelectrode 998, and is soaked in an electrolyte solution inside a spacesurrounded by the film 981 and the film 982 having a depressed portion.

For the film 981 and the film 982 having a depressed portion, a metalmaterial such as aluminum or a resin material can be used, for example.With the use of a resin material for the film 981 and the film 982having a depressed portion, the film 981 and the film 982 having adepressed portion can be changed in their forms when external force isapplied; thus, a flexible secondary battery can be fabricated.

Although FIGS. 10B and 10C illustrate an example where a space is formedby two films, the wound body 993 may be placed in a space formed bybending one film.

When the positive electrode active material described in the aboveembodiment is used in the positive electrode 995, the secondary battery980 with high capacity and excellent cycle characteristics can beobtained.

In FIGS. 10A to 10C, an example in which the secondary battery 980includes a wound body in a space formed by films serving as exteriorbodies is described; however, as illustrated in FIGS. 11A and 11B, asecondary battery may include a plurality of strip-shaped positiveelectrodes, a plurality of strip-shaped separators, and a plurality ofstrip-shaped negative electrodes in a space formed by films serving asexterior bodies, for example.

A laminated secondary battery 500 illustrated in FIG. 11A includes apositive electrode 503 including a positive electrode current collector501 and a positive electrode active material layer 502, a negativeelectrode 506 including a negative electrode current collector 504 and anegative electrode active material layer 505, a separator 507, anelectrolyte solution 508, and an exterior body 509. The separator 507 isprovided between the positive electrode 503 and the negative electrode506 in the exterior body 509. The exterior body 509 is filled with theelectrolyte solution 508. The electrolyte solution described inEmbodiment 2 can be used for the electrolyte solution 508.

In the laminated secondary battery 500 illustrated in FIG. 11A, thepositive electrode current collector 501 and the negative electrodecurrent collector 504 also serve as terminals for an electrical contactwith an external portion. For this reason, the positive electrodecurrent collector 501 and the negative electrode current collector 504may be arranged so as to be partly exposed to the outside of theexterior body 509. Alternatively, a lead electrode and the positiveelectrode current collector 501 or the negative electrode currentcollector 504 may be bonded to each other by ultrasonic welding, andinstead of the positive electrode current collector 501 and the negativeelectrode current collector 504, the lead electrode may be exposed tothe outside of the exterior body 509.

As the exterior body 509 of the laminated secondary battery 500, forexample, a laminate film having a three-layer structure can be employedin which a highly flexible metal thin film of aluminum, stainless steel,copper, nickel, or the like is provided over a film formed of a materialsuch as polyethylene, polypropylene, polycarbonate, ionomer, orpolyamide, and an insulating synthetic resin film of a polyamide-basedresin, a polyester-based resin, or the like is provided over the metalthin film as the outer surface of the exterior body.

FIG. 11B illustrates an example of a cross-sectional structure of thelaminated secondary battery 500. Although FIG. 18A illustrates anexample including only two current collectors for simplicity, an actualbattery includes a plurality of electrode layers.

The example in FIG. 11B includes 16 electrode layers. The laminatedsecondary battery 500 has flexibility even though including 16 electrodelayers. FIG. 11B illustrates a structure including 8 layers of negativeelectrode current collectors 504 and 8 layers of positive electrodecurrent collectors 501, i.e., 16 layers in total. Note that FIG. 11Billustrates a cross section of the lead portion of the negativeelectrode, and the 8 negative electrode current collectors 504 arebonded to each other by ultrasonic welding. It is needless to say thatthe number of electrode layers is not limited to 16, and may be morethan 16 or less than 16. With a large number of electrode layers, thesecondary battery can have high capacity. In contrast, with a smallnumber of electrode layers, the secondary battery can have smallthickness and high flexibility.

FIGS. 12 and 13 each illustrate an example of the external view of thelaminated secondary battery 500. In FIGS. 12 and 13 , the positiveelectrode 503, the negative electrode 506, the separator 507, theexterior body 509, a positive electrode lead electrode 510, and anegative electrode lead electrode 511 are included.

FIG. 14A illustrates external views of the positive electrode 503 andthe negative electrode 506. The positive electrode 503 includes thepositive electrode current collector 501, and the positive electrodeactive material layer 502 is formed on a surface of the positiveelectrode current collector 501. The positive electrode 503 alsoincludes a region where the positive electrode current collector 501 ispartly exposed (hereinafter referred to as a tab region). The negativeelectrode 506 includes the negative electrode current collector 504, andthe negative electrode active material layer 505 is formed on a surfaceof the negative electrode current collector 504. The negative electrode506 also includes a region where the negative electrode currentcollector 504 is partly exposed, that is, a tab region. The areas andthe shapes of the tab regions included in the positive electrode and thenegative electrode are not limited to those illustrated in FIG. 14A.

[Method for Manufacturing Laminated Secondary Battery]

Here, an example of a method for manufacturing the laminated secondarybattery whose external view is illustrated in FIG. 12 will be describedwith reference to FIGS. 14B and 14C.

First, the negative electrode 506, the separator 507, and the positiveelectrode 503 are stacked. FIG. 14B illustrates a stack including thenegative electrode 506, the separator 507, and the positive electrode503. An example described here includes 5 negative electrodes and 4positive electrodes. Next, the tab regions of the positive electrodes503 are bonded to each other, and the tab region of the positiveelectrode on the outermost surface and the positive electrode leadelectrode 510 are bonded to each other. The bonding can be performed byultrasonic welding, for example. In a similar manner, the tab regions ofthe negative electrodes 506 are bonded to each other, and the negativeelectrode lead electrode 511 is bonded to the tab region of the negativeelectrode on the outermost surface.

After that, the negative electrode 506, the separator 507, and thepositive electrode 503 are placed over the exterior body 509.

Subsequently, the exterior body 509 is folded along a dashed line asillustrated in FIG. 14C. Then, the outer edge of the exterior body 509is bonded. The bonding can be performed by thermocompression bonding,for example. At this time, a part (or one side) of the exterior body 509is left unbonded (to provide an inlet) so that the electrolyte solution508 can be introduced later.

Next, the electrolyte solution 508 is introduced into the exterior body509 from the inlet of the exterior body 509. The electrolyte solution508 is preferably introduced in a reduced pressure atmosphere or in aninert gas atmosphere. Lastly, the inlet is bonded. In the above manner,the laminated secondary battery 500 can be manufactured.

When the positive electrode active material described in the aboveembodiment is used in the positive electrode 503, the secondary battery500 with high capacity and excellent cycle characteristics can beobtained.

[Bendable Secondary Battery]

Next, an example of a bendable secondary battery is described withreference to FIGS. 15A, 15B1, 15B2, 15C and 15D and FIGS. 16A and 16B.

FIG. 15A is a schematic top view of a bendable secondary battery 250.FIGS. 15B1, 15B2, and 15C are schematic cross-sectional views takenalong cutting line C1-C2, cutting line C3-C4, and cutting line A1-A2,respectively, in FIG. 15A. The battery 250 includes an exterior body 251and a positive electrode 211 a, and a negative electrode 211 b held inthe exterior body 251. A lead 212 a electrically connected to thepositive electrode 211 a and a lead 212 b electrically connected to thenegative electrode 211 b are extended to the outside of the exteriorbody 251. In addition to the positive electrode 211 a and the negativeelectrode 211 b, an electrolyte solution (not illustrated) is enclosedin a region surrounded by the exterior body 251.

FIGS. 16A and 16B illustrate the positive electrode 211 a and thenegative electrode 211 b included in the battery 250. FIG. 16A is aperspective view illustrating the stacking order of the positiveelectrode 211 a, the negative electrode 211 b, and the separator 214.FIG. 16B is a perspective view illustrating the lead 212 a and the lead212 b in addition to the positive electrode 211 a and the negativeelectrode 211 b.

As illustrated in FIG. 16A, the battery 250 includes a plurality ofstrip-shaped positive electrodes 211 a, a plurality of strip-shapednegative electrodes 211 b, and a plurality of separators 14. Thepositive electrode 211 a and the negative electrode 211 b each include aprojected tab portion and a portion other than the tab. A positiveelectrode active material layer is formed on one surface of the positiveelectrode 211 a other than the tab portion, and a negative electrodeactive material layer is formed on one surface of the negative electrode211 b other than the tab portion.

The positive electrodes 211 a and the negative electrodes 211 b arestacked so that surfaces of the positive electrodes 211 a on each ofwhich the positive electrode active material layer is not formed are incontact with each other and that surfaces of the negative electrodes 211b on each of which the negative electrode active material layer is notformed are in contact with each other.

Furthermore, the separator 214 is provided between the surface of thepositive electrode 211 a on which the positive electrode active materialis formed and the surface of the negative electrode 211 b on which thenegative electrode active material is formed. In FIG. 16A, the separator214 is shown by a dotted line for easy viewing.

In addition, as illustrated in FIG. 16B, the plurality of positiveelectrodes 211 a are electrically connected to the lead 212 a in abonding portion 215 a. The plurality of negative electrodes 211 b areelectrically connected to the lead 212 b in a bonding portion 215 b.

Next, the exterior body 251 is described with reference to FIGS. 15B1,15B2, 15C, and 15D.

The exterior body 251 has a film-like shape and is folded in half withthe positive electrodes 211 a and the negative electrodes 211 b betweenfacing portions of the exterior body 251. The exterior body 251 includesa folded portion 261, a pair of seal portions 262, and a seal portion263. The pair of seal portions 262 is provided with the positiveelectrodes 211 a and the negative electrodes 211 b positionedtherebetween and thus can also be referred to as side seals. The sealportion 263 has portions overlapping with the lead 212 a and the lead212 b and can also be referred to as a top seal.

Part of the exterior body 251 that overlaps with the positive electrodes211 a and the negative electrodes 211 b preferably has a wave shape inwhich crest lines 271 and trough lines 272 are alternately arranged. Theseal portions 262 and the seal portion 263 of the exterior body 251 arepreferably flat.

FIG. 15B1 shows a cross section cut along the part overlapping with thecrest line 271. FIG. 15B2 shows a cross section cut along the partoverlapping with the trough line 272. FIGS. 15B1 and 15B2 correspond tocross sections of the battery 250, the positive electrodes 211 a, andthe negative electrodes 211 b in the width direction.

The distance between an end portion of the negative electrode 211 b inthe width direction and the seal portion 262 is referred to as adistance La. When the battery 250 changes in shape, for example, isbent, the positive electrode 211 a and the negative electrode 211 bchange in shape such that the positions thereof are shifted from eachother in the length direction as described later. At the time, if thedistance La is too short, the exterior body 251 and the positiveelectrode 211 a and the negative electrode 211 b are rubbed hard againsteach other, so that the exterior body 251 is damaged in some cases. Inparticular, when a metal film of the exterior body 251 is exposed, thereis concern that the metal film is corroded by the electrolyte solution.Thus, the distance La is preferably set as long as possible. However, ifthe distance La is too long, the volume of the battery 250 is increased.

The distance La between the end portion of the negative electrode 211 band the seal portion 262 is preferably increased as the total thicknessof the stacked positive electrodes 211 a and negative electrodes 211 bis increased.

Specifically, when the total thickness of the stacked positiveelectrodes 211 a and negative electrodes 211 b and the separators 214(not illustrated) is referred to as a thickness t, the distance La ispreferably 0.8 times or more and 3.0 times or less, further preferably0.9 times or more and 2.5 times or less, still further preferably 1.0times or more and 2.0 times or less as large as the thickness t. Whenthe distance La is in the above-described range, a compact battery whichis highly reliable for bending can be obtained.

Furthermore, when a distance between the pair of seal portions 262 isreferred to as a distance Lb, it is preferable that the distance Lb besufficiently longer than a width Wb of the negative electrode 211 b. Inthis case, even when the positive electrode 211 a and the negativeelectrode 211 b come into contact with the exterior body 251 by changein the shape of the battery 250 such as repeated bending, the positionof part of the positive electrode 211 a and the negative electrode 211 bcan be shifted in the width direction; thus, the positive and negativeelectrodes 211 a and 211 b and the exterior body 251 can be effectivelyprevented from being rubbed against each other.

For example, the difference between the distance Lb (i.e., the distancebetween the pair of seal portions 262) and the width Wb of the negativeelectrode 211 b is preferably 1.6 times or more and 6.0 times or less,further preferably 1.8 times or more and 5.0 times or less, stillfurther preferably 2.0 times or more and 4.0 times or less as large asthe total thickness t of the positive electrode 211 a and the negativeelectrode 211 b.

In other words, the distance Lb, the width Wb, and the thickness tpreferably satisfy the relation of the following Formula 1.

$\begin{matrix}{\frac{{Lb} - {Wb}}{2t} \geq a} & ( {{Formula}1} )\end{matrix}$

In the formula, a is 0.8 or more and 3.0 or less, preferably 0.9 or moreand 2.5 or less, further preferably 1.0 or more and 2.0 or less.

FIG. 15C illustrates a cross section including the lead 212 a andcorresponds to a cross section of the battery 250, the positiveelectrode 211 a, and the negative electrode 211 b in the lengthdirection. As illustrated in FIG. 15C, a space 273 is preferablyprovided between end portions of the positive electrode 211 a and thenegative electrode 211 b in the length direction and the exterior body251 in the folded portion 261.

FIG. 15D is a schematic cross-sectional view of the battery 250 in astate of being bent. FIG. 15D corresponds to a cross section alongcutting line B1-B2 in FIG. 15A.

When the battery 250 is bent, a part of the exterior body 251 positionedon the outer side in bending is unbent and the other part positioned onthe inner side changes its shape as it shrinks. More specifically, thepart of the exterior body 251 positioned on the outer side in bendingchanges its shape such that the wave amplitude becomes smaller and thelength of the wave period becomes larger. In contrast, the part of theexterior body 251 positioned on the inner side in bending changes itsshape such that the wave amplitude becomes larger and the length of thewave period becomes smaller. When the exterior body 251 changes itsshape in this manner, stress applied to the exterior body 251 due tobending is relieved, so that a material itself that forms the exteriorbody 251 does not need to expand and contract. As a result, the battery250 can be bent with weak force without damage to the exterior body 251.

Furthermore, as illustrated in FIG. 15D, when the battery 250 is bent,the positions of the positive electrode 211 a and the negative electrode211 b are shifted relatively. At this time, ends of the stacked positiveelectrodes 211 a and negative electrodes 211 b on the seal portion 263side are fixed by the fixing member 217. Thus, the plurality of positiveelectrodes 211 a and the plurality of negative electrodes 211 b are moreshifted at a position closer to the folded portion 261. Therefore,stress applied to the positive electrode 211 a and the negativeelectrode 211 b is relieved, and the positive electrode 211 a and thenegative electrode 211 b themselves do not need to expand and contract.As a result, the battery 250 can be bent without damage to the positiveelectrode 211 a and the negative electrode 211 b.

Furthermore, the space 273 is provided between the end portions of thepositive and negative electrodes 211 a and 211 b and the exterior body251, whereby the relative positions of the positive electrode 211 a andthe negative electrode 211 b can be shifted while the end portions ofthe positive electrode 211 a and the negative electrode 211 b located onan inner side when the battery 250 is bent do not contact the exteriorbody 251.

In the battery 250 illustrated in FIGS. 15A, 15B1, 15B2, 15C and 15D andFIGS. 16A and 16B, the exterior body, the positive electrode 211 a, andthe negative electrode 211 b are less likely to be damaged and thebattery characteristics are less likely to deteriorate even when thebattery 250 is repeatedly bent and unbent. When the positive electrodeactive material described in the above embodiment is used for thepositive electrode 211 a included in the battery 250, a battery withmore excellent cycle characteristics can be obtained.

(Embodiment 4)

In this embodiment, examples of electronic devices including thesecondary battery of one embodiment of the present invention aredescribed.

First, FIGS. 17A to 17G show examples of electronic devices includingthe bendable secondary battery described in Embodiment 3. Examples of anelectronic device including a flexible secondary battery includetelevision sets (also referred to as televisions or televisionreceivers), monitors of computers or the like, digital cameras ordigital video cameras, digital photo frames, mobile phones (alsoreferred to as cellular phones or mobile phone devices), portable gamemachines, portable information terminals, audio reproducing devices, andlarge game machines such as pachinko machines.

In addition, a flexible secondary battery can be incorporated along acurved inside/outside wall surface of a house or a building or a curvedinterior/exterior surface of an automobile.

FIG. 17A illustrates an example of a mobile phone. A mobile phone 7400is provided with a display portion 7402 incorporated in a housing 7401,an operation button 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400includes a secondary battery 7407. When the secondary battery of oneembodiment of the present invention is used as the secondary battery7407, a lightweight mobile phone with a long lifetime can be provided.

FIG. 17B illustrates the mobile phone 7400 that is bent. When the wholemobile phone 7400 is curved by external force, the secondary battery7407 included in the mobile phone 7400 is also curved. FIG. 17Cillustrates the curved secondary battery 7407. The secondary battery7407 is a thin storage battery. The secondary battery 7407 is curved andfixed. Note that the secondary battery 7407 includes a lead electrode7408 electrically connected to a current collector 7409.

FIG. 17D illustrates an example of a bangle display device. A portabledisplay device 7100 includes a housing 7101, a display portion 7102, anoperation button 7103, and a secondary battery 7104. FIG. 17Eillustrates the bent secondary battery 7104. When the curved secondarybattery 7104 is on a user's arm, the housing changes its form and thecurvature of a part or the whole of the secondary battery 7104 ischanged. Note that the radius of curvature of a curve at a point refersto the radius of the circular arc that best approximates the curve atthat point. The reciprocal of the radius of curvature is curvature.Specifically, part or the whole of the housing or the main surface ofthe secondary battery 7104 is changed in the range of radius ofcurvature from 40 mm to 150 mm When the radius of curvature at the mainsurface of the secondary battery 7104 is greater than or equal to 40 mmand less than or equal to 150 mm, the reliability can be kept high. Whenthe secondary battery of one embodiment of the present invention is usedas the secondary battery 7104, a lightweight portable display devicewith a long lifetime can be provided.

FIG. 17F illustrates an example of a watch-type portable informationterminal. A portable information terminal 7200 includes a housing 7201,a display portion 7202, a band 7203, a buckle 7204, an operation button7205, an input output terminal 7206, and the like.

The portable information terminal 7200 is capable of executing a varietyof applications such as mobile phone calls, e-mailing, viewing andediting texts, music reproduction, Internet communication, and acomputer game.

The display surface of the display portion 7202 is curved, and imagescan be displayed on the curved display surface. In addition, the displayportion 7202 includes a touch sensor, and operation can be performed bytouching the screen with a finger, a stylus, or the like. For example,by touching an icon 7207 displayed on the display portion 7202,application can be started.

With the operation button 7205, a variety of functions such as timesetting, power on/off, on/off of wireless communication, setting andcancellation of a silent mode, and setting and cancellation of a powersaving mode can be performed. For example, the functions of theoperation button 7205 can be set freely by setting the operation systemincorporated in the portable information terminal 7200.

The portable information terminal 7200 can employ near fieldcommunication that is a communication method based on an existingcommunication standard. For example, mutual communication between theportable information terminal 7200 and a headset capable of wirelesscommunication can be performed, and thus hands-free calling is possible.

Moreover, the portable information terminal 7200 includes the inputoutput terminal 7206, and data can be directly transmitted to andreceived from another information terminal via a connector. In addition,charging via the input output terminal 7206 is possible. Note that thecharging operation may be performed by wireless power feeding withoutusing the input output terminal 7206.

The display portion 7202 of the portable information terminal 7200includes the secondary battery of one embodiment of the presentinvention. When the secondary battery of one embodiment of the presentinvention is used, a lightweight portable information terminal with along lifetime can be provided. For example, the secondary battery 7104illustrated in FIG. 17E that is in the state of being curved can beprovided in the housing 7201. Alternatively, the secondary battery 7104illustrated in FIG. 17E can be provided in the band 7203 such that itcan be curved.

A portable information terminal 7200 preferably includes a sensor. Asthe sensor, for example a human body sensor such as a fingerprintsensor, a pulse sensor, or a temperature sensor, a touch sensor, apressure sensitive sensor, an acceleration sensor, or the like ispreferably mounted.

FIG. 17G illustrates an example of an armband display device. A displaydevice 7300 includes a display portion 7304 and the secondary battery ofone embodiment of the present invention. The display device 7300 caninclude a touch sensor in the display portion 7304 and can serve as aportable information terminal.

The display surface of the display portion 7304 is bent, and images canbe displayed on the bent display surface. A display state of the displaydevice 7300 can be changed by, for example, near field communication,which is a communication method based on an existing communicationstandard.

The display device 7300 includes an input output terminal, and data canbe directly transmitted to and received from another informationterminal via a connector. In addition, charging via the input outputterminal is possible. Note that the charging operation may be performedby wireless power feeding without using the input output terminal.

When the secondary battery of one embodiment of the present invention isused as the secondary battery included in the display device 7300, alightweight display device with a long lifetime can be provided.

Next, FIGS. 18A and 18B illustrate an example of a foldable tabletterminal A tablet terminal 9600 illustrated in FIGS. 18A and 18Bincludes a housing 9630 a, a housing 9630 b, a movable portion 9640connecting the housings 9630 a and 9630 b, a display portion 9631, adisplay mode changing switch 9626, a power switch 9627, a power savingmode changing switch 9625, a fastener 9629, and an operation switch9628. A flexible panel is used for the display portion 9631, whereby atablet terminal with a larger display portion can be provided. FIG. 18Aillustrates the tablet terminal 9600 that is opened, and FIG. 18Billustrates the tablet terminal 9600 that is closed.

The tablet terminal 9600 includes a power storage unit 9635 inside thehousings 9630 a and 9630 b. The power storage unit 9635 is providedacross the housings 9630 a and 9630 b, passing through the movableportion 9640.

Part of the display portion 9631 can be a touch panel region and datacan be input when a displayed operation key is touched. A switchingbutton for showing/hiding a keyboard of the touch panel is touched witha finger, a stylus, or the like, so that keyboard buttons can bedisplayed on the display portion 9631.

The display mode switch 9626 can switch the display between a portraitmode and a landscape mode, and between monochrome display and colordisplay, for example. The power saving mode changing switch 9625 cancontrol display luminance in accordance with the amount of externallight in use of the tablet terminal 9600, which is measured with anoptical sensor incorporated in the tablet terminal 9600. Anotherdetection device including a sensor for detecting inclination, such as agyroscope sensor or an acceleration sensor, may be incorporated in thetablet terminal, in addition to the optical sensor.

The tablet terminal is closed in FIG. 18B. The tablet terminal includesthe housing 9630, a solar cell 9633, and a charge and discharge controlcircuit 9634 including a DC-DC converter 9636. The secondary battery ofone embodiment of the present invention is used as the power storageunit 9635.

The tablet terminal 9600 can be folded such that the housings 9630 a and9630 b overlap with each other when not in use. Thus, the displayportion 9631 can be protected, which increases the durability of thetablet terminal 9600. With the power storage unit 9635 including thesecondary battery of one embodiment of the present invention which hashigh capacity and excellent cycle characteristics, the tablet terminal9600 which can be used for a long time for a long period can beprovided.

The tablet terminal illustrated in FIGS. 18A and 18B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, or the time on the display portion, a touch-input function ofoperating or editing data displayed on the display portion by touchinput, a function of controlling processing by various kinds of software(programs), and the like.

The solar cell 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar cell 9633can be provided on one or both surfaces of the housing 9630 and thepower storage unit 9635 can be charged efficiently.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 18B will be described with reference to a blockdiagram in FIG. 18C. The solar cell 9633, the power storage unit 9635,the DC-DC converter 9636, a converter 9637, switches SW1 to SW3, and thedisplay portion 9631 are illustrated in FIG. 18C, and the power storageunit 9635, the DC-DC converter 9636, the converter 9637, and theswitches SW1 to SW3 correspond to the charge and discharge controlcircuit 9634 in FIG. 18B.

First, an example of the operation in the case where power is generatedby the solar cell 9633 using external light is described. The voltage ofelectric power generated by the solar cell is raised or lowered by theDC-DC converter 9636 to a voltage for charging the power storage unit9635. When the power from the solar cell 9633 is used for the operationof the display portion 9631, the switch SW1 is turned on and the voltageof the power is raised or lowered by the converter 9637 to a voltageneeded for operating the display portion 9631. When display on thedisplay portion 9631 is not performed, the switch SW1 is turned off andthe switch SW2 is turned on, so that the power storage unit 9635 can becharged.

Note that the solar cell 9633 is described as an example of a powergeneration means; however, one embodiment of the present invention isnot limited to this example. The power storage unit 9635 may be chargedusing another power generation means such as a piezoelectric element ora thermoelectric conversion element (Peltier element). For example, thepower storage unit 9635 may be charged with a non-contact powertransmission module that transmits and receives power wirelessly(without contact) to charge the battery or with a combination of othercharging means.

FIG. 19 illustrates other examples of electronic devices. In FIG. 19 , adisplay device 8000 is an example of an electronic device including asecondary battery 8004 of one embodiment of the present invention.Specifically, the display device 8000 corresponds to a display devicefor TV broadcast reception and includes a housing 8001, a displayportion 8002, speaker portions 8003, the secondary battery 8004, and thelike. The secondary battery 8004 of one embodiment of the presentinvention is provided in the housing 8001. The display device 8000 canreceive electric power from a commercial power supply. Alternatively,the display device 8000 can use electric power stored in the secondarybattery 8004. Thus, the display device 8000 can operate with the use ofthe secondary battery 8004 of one embodiment of the present invention asan uninterruptible power supply even when electric power cannot besupplied from a commercial power supply due to power failure or thelike.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoretic displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 8002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like other than TV broadcast reception.

In FIG. 19 , an installation lighting device 8100 is an example of anelectronic device using a secondary battery 8103 of one embodiment ofthe present invention. Specifically, the lighting device 8100 includes ahousing 8101, a light source 8102, the secondary battery 8103, and thelike. Although FIG. 19 illustrates the case where the secondary battery8103 is provided in a ceiling 8104 on which the housing 8101 and thelight source 8102 are installed, the secondary battery 8103 may beprovided in the housing 8101. The lighting device 8100 can receiveelectric power from a commercial power supply. Alternatively, thelighting device 8100 can use electric power stored in the secondarybattery 8103. Thus, the lighting device 8100 can operate with the use ofthe secondary battery 8103 of one embodiment of the present invention asan uninterruptible power supply even when electric power cannot besupplied from a commercial power supply due to power failure or thelike.

Note that although the installation lighting device 8100 provided in theceiling 8104 is illustrated in FIG. 19 as an example, the secondarybattery of one embodiment of the present invention can be used as aninstallation lighting device provided in, for example, a wall 8105, afloor 8106, a window 8107, or the like other than the ceiling 8104.Alternatively, the secondary battery can be used in a tabletop lightingdevice or the like.

As the light source 8102, an artificial light source which emits lightartificially by using power can be used. Specifically, an incandescentlamp, a discharge lamp such as a fluorescent lamp, and a light-emittingelement such as an LED or an organic EL element are given as examples ofthe artificial light source.

In FIG. 19 , an air conditioner including an indoor unit 8200 and anoutdoor unit 8204 is an example of an electronic device including asecondary battery 8203 of one embodiment of the present invention.Specifically, the indoor unit 8200 includes a housing 8201, an airoutlet 8202, the secondary battery 8203, and the like. Although FIG. 19illustrates the case where the secondary battery 8203 is provided in theindoor unit 8200, the secondary battery 8203 may be provided in theoutdoor unit 8204. Alternatively, the secondary batteries 8203 may beprovided in both the indoor unit 8200 and the outdoor unit 8204. The airconditioner can receive electric power from a commercial power supply.Alternatively, the air conditioner can use electric power stored in thesecondary battery 8203. Particularly in the case where the secondarybatteries 8203 are provided in both the indoor unit 8200 and the outdoorunit 8204, the air conditioner can operate with the use of the secondarybattery 8203 of one embodiment of the present invention as anuninterruptible power supply even when electric power cannot be suppliedfrom a commercial power supply due to power failure or the like.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 19 as an example, thesecondary battery of one embodiment of the present invention can be usedin an air conditioner in which the functions of an indoor unit and anoutdoor unit are integrated in one housing.

In FIG. 19 , an electric refrigerator-freezer 8300 is an example of anelectronic device using a secondary battery 8304 of one embodiment ofthe present invention. Specifically, the electric refrigerator-freezer8300 includes a housing 8301, a refrigerator door 8302, a freezer door8303, the secondary battery 8304, and the like. The secondary battery8304 is provided in the housing 8301 in FIG. 19 . The electricrefrigerator-freezer 8300 can receive electric power from a commercialpower supply. Alternatively, the electric refrigerator-freezer 8300 canuse electric power stored in the secondary battery 8304. Thus, theelectric refrigerator-freezer 8300 can operate with the use of thesecondary battery 8304 of one embodiment of the present invention as anuninterruptible power supply even when electric power cannot be suppliedfrom a commercial power supply due to power failure or the like.

In addition, in a time period when electronic devices are not used,particularly when the proportion of the amount of power which isactually used to the total amount of power which can be supplied from acommercial power source (such a proportion referred to as a usage rateof power) is low, power can be stored in the secondary battery, wherebythe usage rate of power can be reduced in a time period when theelectronic devices are used. For example, in the case of the electricrefrigerator-freezer 8300, power can be stored in the secondary battery8304 in night time when the temperature is low and the refrigerator door8302 and the freezer door 8303 are not often opened and closed. On theother hand, in daytime when the temperature is high and the refrigeratordoor 8302 and the freezer door 8303 are frequently opened and closed,the secondary battery 8304 is used as an auxiliary power source; thus,the usage rate of power in daytime can be reduced.

According to one embodiment of the present invention, the secondarybattery can have excellent cycle characteristics. Furthermore, inaccordance with one embodiment of the present invention, a secondarybattery with high capacity can be obtained; thus, the secondary batteryitself can be made more compact and lightweight. Thus, the secondarybattery of one embodiment of the present invention is used in theelectronic device described in this embodiment, whereby a morelightweight electronic device with a longer lifetime can be obtained.This embodiment can be implemented in appropriate combination with anyof the other embodiments.

(Embodiment 5)

In this embodiment, examples of vehicles including the secondary batteryof one embodiment of the present invention are described.

The use of secondary batteries in vehicles enables production ofnext-generation clean energy vehicles such as hybrid electric vehicles(HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles(PHEVs).

FIGS. 20A to 20C each illustrate an example of a vehicle using thesecondary battery of one embodiment of the present invention. Anautomobile 8400 illustrated in FIG. 20A is an electric vehicle that runson the power of an electric motor. Alternatively, the automobile 8400 isa hybrid electric vehicle capable of driving appropriately using eitheran electric motor or an engine. One embodiment of the present inventioncan provide a high-mileage vehicle. The automobile 8400 includes thesecondary battery. As the secondary battery, the modules of thesecondary batteries illustrated in FIGS. 12C and 12D may be arranged tobe used in a floor portion in the automobile. Alternatively, a batterypack in which a plurality of secondary batteries each of which isillustrated in FIGS. 17A to 17C are combined may be placed in a floorportion in the automobile. The secondary battery is used not only fordriving an electric motor 8406, but also for supplying electric power toa light-emitting device such as a headlight 8401 or a room light (notillustrated).

The secondary battery can also supply electric power to a display deviceof a speedometer, a tachometer, or the like included in the automobile8400. Furthermore, the secondary battery can supply electric power to asemiconductor device included in the automobile 8400, such as anavigation system.

FIG. 20B illustrates an automobile 8500 including the secondary battery.The automobile 8500 can be charged when the secondary battery issupplied with electric power through external charging equipment by aplug-in system, a contactless power feeding system, or the like. In FIG.20B, a secondary battery 8024 included in the automobile 8500 is chargedwith the use of a ground-based charging apparatus 8021 through a cable8022. In charging, a given method such as CHAdeMO (registered trademark)or Combined Charging System may be employed as a charging method, thestandard of a connector, or the like as appropriate. The ground-basedcharging apparatus 8021 may be a charging station provided in a commercefacility or a power source in a house. With the use of a plug-intechnique, the secondary battery 8024 included in the automobile 8500can be charged by being supplied with electric power from the outside,for example. The charging can be performed by converting AC electricpower into DC electric power through a converter such as an AC-DCconverter.

Furthermore, although not illustrated, the vehicle may include a powerreceiving device so that it can be charged by being supplied withelectric power from an above-ground power transmitting device in acontactless manner. In the case of the contactless power feeding system,by fitting a power transmitting device in a road or an exterior wall,charging can be performed not only when the electric vehicle is stoppedbut also when driven. In addition, the contactless power feeding systemmay be utilized to perform transmission and reception of electric powerbetween vehicles. A solar cell may be provided in the exterior of theautomobile to charge the secondary battery when the automobile stops ormoves. To supply electric power in such a contactless manner, anelectromagnetic induction method or a magnetic resonance method can beused.

FIG. 20C shows an example of a motorcycle using the secondary battery ofone embodiment of the present invention. A motor scooter 8600illustrated in FIG. 20C includes a secondary battery 8602, side mirrors8601, and indicators 8603. The secondary battery 8602 can supplyelectric power to the indicators 8603.

Furthermore, in the motor scooter 8600 illustrated in FIG. 20C, thesecondary battery 8602 can be held in a storage unit under seat 8604. Itis preferable that the secondary battery 8602 can be held in the storageunit under seat 8604 even with a small size. It is preferable that thesecondary battery 8602 is detachable; thus, the secondary battery 8602is carried indoors when it is charged, and is stored before themotorcycle is driven.

In accordance with one embodiment of the present invention, thesecondary battery can have improved cycle characteristics and thecapacity of the secondary battery can be increased. Thus, the secondarybattery itself can be made more compact and lightweight. The compact andlightweight secondary battery contributes to a reduction in the weightof a vehicle, and thus increases the driving radius. Furthermore, thesecondary battery included in the vehicle can be used as a power sourcefor supplying electric power to products other than the vehicle. In sucha case, the use of a commercial power source can be avoided at peak timeof electric power demand, for example. If the use of a commercial powersource can be avoided at peak time of electric power demand, theavoidance can contribute to energy saving and a reduction in carbondioxide emissions. Moreover, if the cycle characteristics are excellent,the secondary battery can be used for a long period; thus, the useamount of rare metals such as cobalt can be reduced.

This embodiment can be implemented in appropriate combination with theother embodiments.

[Example 1]

In this example, cobalt was used for a transition metal contained in afirst region of a positive electrode active material. Then, the positiveelectrode active material prepared by adding magnesium and fluorine tothe starting material, and a positive electrode active material preparedwithout adding magnesium and fluorine as a comparative example wereprepared and their characteristics were analyzed. Also, the cyclecharacteristics were evaluated by changing the concentration ofmagnesium and fluorine added to the starting material.

<Fabrication of Samples 1 to 6 of Positive Electrode Active Material>

Samples 1 to 6 of positive electrode active materials were prepared withvarying concentrations of magnesium source and fluorine source. Lithiumcarbonate and cobalt oxide were used as common starting materials.Magnesium oxide and lithium fluoride were used as addition startingmaterials different from each sample.

In the sample 1, magnesium oxide and lithium fluoride were added to thestarting material so as to contain 0.5 atomic % of magnesium and 1atomic % of fluorine with respect to cobalt contained in the commonstarting material. Hereinafter, this is referred to as “in the sample 1,0.5 mol % MgO and 1 mol % LiF were used as an addition startingmaterial”.

As described above, in this specification and the like, the amount ofthe addition starting material is indicated by atomic % or mol % withrespect to the transition metal contained in the common startingmaterial. The same applies to the sample 2 and the following.

For the sample 2, 0.5 mol % MgO and 0.5 mol % LiF with respect to cobaltwere used as addition starting materials. For the sample 3, 0.5 mol %MgO and 2 mol % LiF were used as addition starting materials. For thesample 4 which is a comparative example, 1 mol % LiF was used as anaddition starting material, and magnesium was not added. For the sample5 which is a comparative example, 0.5 mol % MgO was used as an additionstarting material, and fluorine was not added. For the sample 6 which isa comparative example, neither magnesium nor fluorine was added. Thecommon starting materials and the addition starting materials for eachsample are shown in Table 1.

TABLE 1 Common Addition Mg:F starting starting (atomic material materialratio) Sample 1 Li₂CO₃ 0.5 mol % MgO, 1:2 Co₃O₄ 1 mol % LiF Sample 2 0.5mol % MgO, 1:1 0.5 mo % LiF Sample 3 0.5 mol % MgO, 1:4 2 mol % LiFSample 4 1 mol % LiF — (comparative example) Sample 5 0.5 mol % MgO —(comparative example) Sample 6 — — (comparative example)

A positive electrode active material was obtained from each of the abovesix samples in the following process, as in the manufacturing methoddescribed in Embodiment 1: mixing of starting materials, first heating,cooling, sieving, second heating, cooling, and recovery. For theparticles in the middle of this process and the positive electrodeactive material after the process, the following analysis was carriedout.

<STEM-EDX>

For the sample 1 and the sample 5 (comparative example), a section nearthe surface of the particle before the second heating was analyzed usingSTEM-EDX. FIGS. 22A to 22C show STEM-EDX images of the sample 1 beforethe second heating. FIGS. 23A to 23C show STEM-EDX images of the sample5 (comparative example) before the second heating. FIGS. 22A and 23Ashow STEM images. FIGS. 22B and 23B show magnesium mappings. FIGS. 22Cand 23C show fluorine mappings.

As shown in FIG. 22B, in the sample 1 containing magnesium and fluorineas starting materials, it was observed that magnesium segregated to someextent near the surface of the particles before performing the secondheating. The segregated region was about 1 nm to 2 nm from the surfaceof the particles.

On the other hand, as shown in the EDX mapping in FIG. 23B, in thesample 5 containing magnesium as a starting material but containing nofluorine, no segregation of magnesium into the vicinity of the surfacewas observed.

Note that as shown in FIG. 22C and FIG. 23C, almost no fluorine wasobserved in an inner portion of the positive electrode active materialin both of the sample 1 and the sample 5. This is thought to be becauseEDX is hard to detect fluorine which is a lightweight element.

<X-Ray Photoelectron Spectroscopy (XPS)>

Next, for the sample 1 and the sample 5 (comparative example), theamount of magnesium near the surface of the positive electrode activematerial before and after the second heating was analyzed.

The XPS analysis conditions were as follows.

-   -   Measurement device: Quantera II manufactured by PHI, Inc.    -   X-ray source: monochromated A1 (1486.6 eV)    -   Detection area: 100 μm φ    -   Detection depth: about 4 to 5 nm (extraction angle 45°)    -   Measurement spectrum: wide, Li1s, Co2p, Ti2p, O1s, C1s, F1s,        S2p, Ca2p, Mg1s, Na1s, Zr3d

The results of quantifying the concentration of each element using XPSare shown in Table 2. The quantitative accuracy is about ±1 atomic %.The lower limit of detection is about 1 atomic %, depending on theelement. In Ca, the Mg Auger peak separated by waveform is removed, sothe quantitative error is larger than usual.

Table 3 shows the calculation results of the existence ratio of eachelement when cobalt is 1.

TABLE 2 Quantitative value (atomic %) Li Co O C Sample 5 15.000 16.60047.800 17.600 Before second heating Sample 5 16.600 15.700 48.200 16.600After second heating Sample 1 12.300 18.400 47.200 16.400 Before secondheating Sample 1 12.900 16.700 47.400 16.500 After second heating F S CaMg Na Zr Sample 5 0.000 0.000 0.400 0.000 2.000 0.500 Before secondheating Sample 5 0.000 0.000 0.400 0.000 2.100 0.400 After secondheating Sample 1 0.800 0.000 0.500 2.800 1.300 0.400 Before secondheating Sample 1 0.000 0.000 0.400 3.000 2.600 0.600 After secondheating

TABLE 3 Existence ratio Li Co O C Sample 5 0.904 1.000 2.880 1.060Before second heating Sample 5 1.057 1.000 3.070 1.057 After secondheating Sample 1 0.668 1.000 2.565 0.891 Before second heating Sample 10.772 1.000 2.838 0.988 After second heating F S Ca Mg Na Zr Sample 50.000 0.000 0.024 0.000 0.120 0.030 Before second heating Sample 5 0.0000.000 0.025 0.000 0.134 0.025 After second heating Sample 1 0.043 0.0000.027 0.152 0.071 0.022 Before second heating Sample 1 0.000 0.000 0.0240.180 0.156 0.036 After second heating

Among the existence ratios of the elements shown in Table 3, a graph ofmagnesium is shown in FIG. 24 .

As shown in Table 2, Table 3, and FIG. 24 , in the sample 1 containingmagnesium and fluorine as addition starting materials, magnesium waspresent near the surface of the positive electrode active materialmeasurable by XPS even before the second heating. After the secondheating, the amount of magnesium near the surface of the positiveelectrode active material further increased.

In other words, it seems that segregation of magnesium on the surface ofthe positive electrode active material advanced by the second heating.Thus, it was confirmed that the positive electrode active material ofthe sample 1 has a first region in the inner portion and a second regionin the superficial portion, and that the first region contains lithiumcobalt oxide and the second region contains magnesium.

On the other hand, in the sample 5 containing no fluorine as an additionstarting material and containing only magnesium, the amount of magnesiumnear the surface of the positive electrode active material was lowerthan the detection lower limit both before and after the second heating.In other words, fluorine contained in the starting material surprisinglyproved to have the effect of segregating magnesium in the superficialportion of the positive electrode active material.

<Cycle Characteristics>

Next, CR2032 (diameter 20 mm, height 3.2 mm) coin type secondarybatteries were fabricated using a positive electrode active material ofthe sample 1 before and after the second heating, a positive electrodeactive material of the sample 5 before and after the first heating, andpositive electrode active materials of the sample 2, sample 3, sample 4,and sample 6. Their cycle characteristics were evaluated.

A positive electrode formed by applying slurry in which the formedpositive electrode active material, acetylene black (AB), andpolyvinylidene fluoride (PVDF) were mixed at a weight ratio of positiveelectrode active material:AB:PVDF=95:2.5:2.5 to a current collector wasused for the positive electrode.

A lithium metal was used for the counter electrode.

As an electrolyte contained in an electrolyte solution, 1 mol/L lithiumhexafluorophosphate (LiPF₆) was used, and as the electrolyte solution, asolution in which ethylene carbonate (EC) and diethyl carbonate (DEC) ata volume ratio of EC:DEC=3:7 and vinylene carbonate (VC) at a 2 weight %were mixed was used.

A positive electrode can and a negative electrode can were formed ofstainless steel (SUS).

The measurement temperature for the cycle characteristics test was 25°C. Charging was carried out at a constant current with a current densityof 68.5 mA/g per active material weight and an upper limit voltage of4.6 V, followed by constant voltage charge until a current density wasreached to 1.4 mA/g. Discharge was carried out at a constant currentwith a current density of 68.5 mA/g per active material weight and witha lower limit voltage of 2.5 V. 30 cycles of charging and dischargingwere performed.

FIGS. 25A and 25B are graphs showing the cycle characteristics of thesecondary battery using the positive electrode active material beforeand after the second heating of the sample 1 and before and after thefirst heating of the sample 5. FIG. 25A is an energy density at 4.6 Vcharging. FIG. 25B is a graph of energy density retention rate at 4.6 Vcharging. The energy density is the product of the discharge capacityand the discharge average voltage.

As shown in FIGS. 25A and 25B, in the sample 1 containing magnesium andfluorine added as the starting material, cycle performance was greatlyimproved by performing the second heating. The energy density was alsofavorable.

This is considered to be due to the fact that the amount of magnesiumexisting in the vicinity of the surface of the positive electrode activematerial was increased by performing the second heating, as is clearfrom the above XPS results.

On the other hand, in Sample 5 containing only magnesium as an additionstarting material, no significant difference in cycle characteristicswas observed before and after the second heating.

FIGS. 26 and 27 show graphs of the cycle characteristics of thesecondary battery using the positive electrode active material of thesample 1 to the sample 6 after the second heating. FIG. 26 is a graph ofenergy density at 4.6 V charging, and FIG. 27 is a graph of energydensity retention rate at 4.6 V charging.

As shown in FIG. 26 and FIG. 27 , the sample 4 (comparative example) inwhich only fluorine was added to the starting material and the sample 5(comparative example) in which only magnesium was added showed cyclecharacteristics inferior to the sample 6 (comparative example) in whichneither magnesium nor fluorine was added.

On the other hand, the samples 1 to 3 in which magnesium and fluorinewere added to the starting material showed good cycle characteristics.The best cycle characteristics were shown in the sample 1 in which theatomic ratio of magnesium to fluorine was 1:2. The sample 2 having acontent ratio of magnesium to fluorine of 1:4 showed good cyclecharacteristics. As is clear from FIG. 26 , not only the cyclecharacteristics but also the energy density was good.

It was revealed that a positive electrode active material exhibitinggood cycle characteristics can be obtained by adding magnesium andfluorine to the starting material in this way. Further, the atomic ratioof magnesium to fluorine contained in the starting material ispreferably Mg:F=1:x (1.5≤x≤4), and it became clear that Mg:F=about 1:2is most preferable.

<Fabrication of Positive Electrode Active Materials of Samples 7 And 8>

Next, a positive electrode active material of the samples 7 and 8, inwhich the amount of addition was changed while keeping the ratio betweenmagnesium and fluorine constant (Mg:F=1:2), was prepared.

For the sample 7, 1 mol % MgO and 2 mol % LiF were used as additionstarting materials. For the sample 8, 2 mol % MgO and 4 mol % LiF wereused as addition starting materials. For each of the sample 7 and thesample 8, the starting materials were mixed and subjected to the firstheating, cooled, sieved, subjected to the second heating, cooled, andcollected in the same manner as in the manufacturing method described inEmbodiment 1. In this way, a positive electrode active material and asecondary battery were fabricated.

Table 4 shows common starting materials and addition starting materialsof the sample 1, the sample 7, and the sample 8, in which the atomicratio of magnesium to fluorine as raw material is Mg:F=1:2, and of thesample 6 without addition of magnesium and fluorine as a comparativeexample.

TABLE 4 Common Addition Mg:F starting starting (atomic material materialratio) Sample 1 Li₂CO₃ 0.5 mol % MgO, 1:2 Co₃O₄ 1 mol % LiF Sample 7 1mol % MgO, 1:2 2 mol % LiF Sample 8 2 mol % MgO, 1:2 4 mol % LiF Sample6 1 mol % LiF — (comparative example)

<Cycle Characteristics>

FIGS. 28A and 28B are graphs of cycle characteristics of secondarybatteries using positive electrode active materials of the sample 1, thesample 7, the sample 8, and the sample 6 (comparative example). FIG. 28Ais a graph of energy density at 4.6 V charging, and FIG. 28B is a graphof energy density retention rate at 4.6 V charging.

As shown in FIG. 28A and FIG. 28B, all samples in which the atomicnumber ratio of magnesium to fluorine of Mg:F=1:2 as a raw materialshowed good cycle characteristics.

Among them, the sample 7 using 1 mol % MgO and 2 mol % LiF as additionstarting materials showed the best cycle characteristics, and the energydensity retention rate after 30 cycles was 93%. Also as apparent fromFIG. 28A, not only the cycle characteristics but also the energy densitywas good.

[Example 2]

This example shows a comparison result of a positive electrode activematerial having a second region formed by segregation of magnesium and apositive electrode active material having a magnesium oxide layer formedby coating from the outside.

<Positive Electrode Active Material Having Second Region Formed bySegregation>

The sample 7 in Example 1 was used as a positive electrode activematerial having a second region formed by segregation of magnesium, inwhich 1 mol % MgO and 2 mol % LiF were used as addition startingmaterials.

<Positive Electrode Active Material Having MgO Formed by Coating fromOutside>

As a positive electrode active material having a magnesium oxide layerformed by coating from outside, positive electrode active materials ofthe sample 9 (comparative example) and the sample 10 (comparativeexample) in which lithium cobalt oxide was coated with magnesium oxideusing polygonal barrel sputtering were used. Manufacturing methods ofthe sample 9 (comparative example) and the sample 10 (comparativeexample) are described below.

Lithium cobalt oxide produced by NIPPON CHEMICAL INDUSTRIAL CO., LTD.(product name: C-10N) was used. For polygonal barrel sputtering,magnesium oxide was used as a target, power was set to 450 W, Ar and O₂were used as sputtering gas to form a film. The partial pressures of Arand O₂ were set at 0.6 Pa and 0.5 Pa, respectively. The processing timewas 36 minutes for the sample 9 and 180 minutes for the sample 10.

STEM observation after polygonal barrel sputtering treatment revealedthat a magnesium oxide layer of about 1 nm to 3 nm adhered to thesurface of the positive electrode active material of the sample 9. Inthe sample 10, a magnesium oxide layer of about 6 nm to 8 nm adhered tothe surface of the positive electrode active material.

After that, the sample 9 and the sample 10 were heated at 800° C. for 2hours in the same manner as in the second heating described inEmbodiment 1. Dry air having a temperature rise of 200° C./h and a dewpoint of −109° C. was flowed at 10 L/min.

The conditions for the sample 7, the sample 9 (comparative example), andthe sample 10 (comparative example) to be compared in this example areshown in Table 5.

TABLE 5 Inner Superficial portion portion Sample 7 LiCoO₂ Second regionformed by segregation, about 1 nm Sample 9 MgO formed by barrel-(comparative example) sputtering, about 3 nm Sample 10 MgO formed bybarrel- (comparative example) sputtering, about 8 nm

<STEM>

Cross sections of the positive electrode active materials of the sample7 and the sample 10 (comparative example) were observed using STEM.FIGS. 29A and 29B show STEM images of the sample 7 having a secondregion formed by segregation. FIGS. 30A and 30B show STEM images of thesample 10 (comparative example) having a magnesium oxide layer formed bycoating from the outside.

In the sample 7, it was possible to observe that the first region andthe second region were different regions from the difference in imagebrightness or the like. As shown in FIGS. 29A and 29B, in the sample 7having the second region formed by segregation, the second region ofabout 1 nm to 2 nm was observed.

Also in the sample 10 (comparative example), as shown in FIGS. 30A and30B, it was possible to observe that a magnesium oxide layer was formedon lithium cobalt oxide from the difference in image brightness or thelike. In the sample 10 (comparative example), the magnesium oxide layerof about 8 nm was observed.

In both of the sample 7 and the sample 10 (comparative example), atleast part of the arrangement of cations and anions between differentlayers is aligned, and orientations of the crystal in the first regionand the crystal in the second region were aligned.

<Charge and Discharge Characteristics>

Secondary batteries were fabricated in the same manner as in Example 1using the positive electrode active materials of the sample 7, thesample 9 (comparative example), and the sample 10 (comparative example),and charge and discharge characteristics were evaluated. FIGS. 31A, 31B,and 31C show the charge and discharge characteristics of the secondarybattery using the positive electrode active material of the sample 7,the sample 9 (comparative example), and the sample 10 (comparativeexample), respectively. 10 [0330]

As shown in FIGS. 31A to 31C, the sample 7 having the second regionformed by the segregation of magnesium had larger capacity than thesamples 9 and 10 having the magnesium oxide layer formed by polygonalbarrel sputtering, and the sample 7, and exhibited better cyclecharacteristics.

Next, the results of evaluating the cycle characteristics of thesecondary batteries using the positive electrode active materials of thesample 7, the sample 9 (comparative example), and the sample 10(comparative example) are shown in FIGS. 32A and 32B. The cyclecharacteristic test was carried out in the same manner as in Example 1.

FIG. 32A is a graph of an energy density at 4.6 V charging, and FIG. 32Bis a graph of an energy density retention rate at 4.6 V charging. Asshown in FIG. 32B, the sample 7 having the second region formed bysegregation has much better cycle characteristics than the sample 9 andthe sample 10 each having the magnesium oxide layer formed by thepolygonal barrel sputtering. As shown in FIG. 32A, the energy densitywas also better in the sample 7.

Thus, it was revealed that the second region formed by the segregationof magnesium contributes to better charge and discharge characteristicsand cycle characteristics than the magnesium oxide layer formed by thepolygonal barrel sputtering.

From these results, it was presumed that the magnesium-containing regionformed as a result of segregation of magnesium previously contained inthe starting material on the surface contributes to the stabilization ofthe crystal structure of lithium cobalt oxide rather than the magnesiumoxide layer coated from the outside of the lithium cobalt oxideparticles.

[Example 3]

In this example, characteristics of the positive electrode activematerial having a second region formed by segregation of magnesium wereclarified by various analyses.

<Analyzed Positive Electrode Active Material>

The sample 7 of Example 1 using 1 mol % MgO and 2 mol % LiF as additionstarting materials was used as an analysis sample of this example.

<STEM, FFT>

FIGS. 33A to 33C and FIGS. 34A to 34C show STEM-FFT images of the crosssection of the vicinity of the surface of the positive electrode activematerial of the sample 7 having the second region formed by segregation.FIG. 33A is a STEM image of the vicinity of the surface of the positiveelectrode active material, and FIG. 33B is the FFT (Fast FourierTransform) image of the region indicated by FFT1 in FIG. 33A. Someluminescent spots of the FFT image of FIG. 33B are called A, B, C, O asshown in FIG. 33C.

Regarding the luminescent spots of the FFT image in the region indicatedby FFT1, the measured values were d=0.20 nm for OA, d=0.24 nm for OB,and d=0.25 nm for OC. In addition, ∠AOB=53°, ∠BOC=74°, and ∠AOC=127°.

The results are close to d=0.21 nm for OA(200), d=0.24 nm for OB(1-11),d=0.24 nm for OC(-1-11), ∠AOB=55°, ∠BOC=70°, and ∠AOC=125° which areobtained from magnesium oxide (MgO) data (ICDD45-0945) in the ICDD(International Centre for Diffraction Data). Therefore, it became clearthat the region indicated by FFT1 is a region having a rock salt typecrystal structure and is an image of [011] incidence.

FIG. 34A is a STEM image of the vicinity of the surface of the positiveelectrode active material of FIG. 33A. FIG. 34B shows the FFT image ofthe region indicated by FFT2 in FIG. 34A. Some luminescent spot of theFFT image of FIG. 34B is called A, B, C, O as shown in FIG. 34C.

Regarding the luminescent spots of the FFT image in the region indicatedby FFT2, the measured values were d=0.24 nm for OA, d=0.20 nm for OB,and d=0.45 nm for OC. In addition,  AOB=25°, ∠BOC=53°, and ∠AOC=78°.

The results are close to d=0.24 nm for OA(101), d=0.20 nm for OB(104),d=0.47 nm for OC(003), ∠AOB=25°, ∠BOC=55°, and ∠AOC=80° which areobtained from lithium cobalt oxide (LiCoO₂) data (ICDD50-0653) in theICDD database. Therefore, it became clear that the region indicated byFFT2 is a region containing lithium cobalt oxide and is an image of[010] incidence.

Furthermore, from the STEM images of FIG. 33A and FIG. 34A, it wasobserved that the brightness of the image was different between thefirst region and the second region, and that orientations of the crystalin the first region and the crystal in the second region were aligned.

<STEM-EDX>

Next, analysis results of the vicinity of the surface and the vicinityof crystal defects of the sample 7 using STEM-EDX are shown in FIGS. 35Ato 35C, FIG. 36 , and FIGS. 37A1 to 37B2.

FIGS. 35A to 35C show the STEM-EDX analysis results of the vicinity ofthe surface of the positive electrode active material of the sample 7.FIG. 35A shows the STEM image, FIG. 35B shows the magnesium mapping, andFIG. 35C shows the fluorine mapping.

Magnesium in the vicinity of the surface of the positive electrodeactive material was observed more clearly in the sample 7 (FIGS. 35A to35C) in which 1 mol % MgO and 2 mol % LiF were added as startingmaterials than in the sample 1 (FIGS. 22A to 22C) in which 0.5 mol % MgOand 1 mol % LiF were added as starting materials described in Example 1.This supports the result of Example 1 that the larger the amount ofmagnesium in the vicinity of the surface of the positive electrodeactive material is, the better the cycle characteristic is.

FIG. 36 is a cross-sectional TEM image of the vicinity of the crystaldefects of the positive electrode active material of the sample 7. In acrystal defect 1001 in FIG. 36 , portions, which seem to be crystaldefects, with different brightness from other portions were observed.

FIGS. 37A1 to 37B2 show STEM-EDX analysis results of the crystal defect1001 in FIG. 36 .

FIG. 37A1 shows a STEM image of the crystal defect 1001. FIG. 37A2 showsthe magnesium mapping. FIG. 37B1 shows the fluorine mapping. FIG. 37B2shows the zirconium mapping.

As shown in FIG. 37A2, segregation of magnesium was observed in thecrystal defect of the positive electrode active material of the sample 7and its vicinity. This shows that the sample 7 is the positive electrodeactive material having a second region not only in the vicinity of thesurface but also in the inner portion. In addition, as shown in FIG.37B2, zirconium segregation was also observed in the inner portion ofthe second region. Since the step of mixing the starting materials iscarried out by a ball mill and zirconium is used for the material of theball mill, there is a possibility that zirconium enters the sample 7.Moreover, as shown in FIG. 37B1, fluorine is hardly detected in theinner portion of the second region. This is probably because fluorine,which is a lightweight element, is difficult to detect with EDX.

<ToF-SIMS>

Next, the positive electrode active material of the sample 7 having thesecond region formed by segregation is analyzed using ToF-SIMS toinvestigate the distribution of magnesium and fluorine in the depthdirection, and the analysis results are shown in FIG. 38 .

Analysis was carried out in the depth direction from the surface of thepositive electrode active material by alternately repeating ToF-SIMSanalysis and sputtering using particles of multiple positive electrodeactive materials as samples. TOF.SIMS 5-300 (manufactured by ION-TOFGmbH) was used as the measuring device. Cs was used as an ion source forsputtering. The analysis was conducted in the range of about 50 μmsquare.

FIG. 38 shows a graph of the intensity of magnesium oxide ion ([MgO₂]²⁻)and fluoride ion (F⁻). The horizontal axis shows the number ofmeasurements (the number of cycles). Since the analysis on negative ionswas carried out in this measurement, the distribution of magnesium wasevaluated by the intensity of [MgO₂]²⁻. Note that each intensity wasnormalized by setting the maximum value to 1.

As shown in FIG. 38 , it was revealed that in the sample 7 having thesecond region formed by segregation of magnesium, the distribution ofmagnesium and fluorine in the depth direction overlapped with the peak.

<XPS>

Next, Table 6 and FIG. 39 show the results of analyzing the positiveelectrode active material of the sample 7 using XPS before and after thesecond heating. The XPS analysis was carried out in the same manner asin Example 1.

The results of quantifying the concentration of each element of thesample 7 using XPS are shown in Table 6. Note that the quantitativeaccuracy is about ±1 atomic %, and the detection lower limit is about 1atomic % depending on the element. In Ca, the curve-fitting Mg Augerpeak is removed, and the quantitative error is larger than usual.

TABLE 6 Quantitative value (atomic %) Li Co Ti O C F S Ca Mg Na ZrSample 7 12.2 17.9 0.0 47.9 14.5 1.6 0.0 0.4 3.7 1.4 0.3 Before secondheating Sample 7 12.5 15.9 0.0 46.6 14.7 1.4 0.2 0.7 5.5 2.2 0.3 Aftersecond heating

The quantitative values in Table 6 are values in the range of from 4 nmto 5 nm in depth from the surface of the positive electrode activematerial toward the center, which can be analyzed by XPS, and when thetotal amount of lithium, cobalt, titanium, oxygen, carbon, fluorine,sulfur, calcium, magnesium, sodium, and zirconium is taken as 100 atomic%.

As shown in Table 6, in the range of from 4 nm to 5 nm in depth from thesurface toward the center of the sample 7 having the second regionformed by segregation after the second heating, the magnesiumconcentration was 5.5 atomic % and the fluorine concentration was 1.4atomic % when the total amount of lithium, cobalt, titanium, oxygen,carbon, fluorine, sulfur, calcium, magnesium, sodium, and zirconium wastaken as 100%.

When the total amount of lithium, cobalt, oxygen, fluorine, andmagnesium was taken as 100%, the concentration of magnesium wascalculated to be 6.7%, and the concentration of fluorine was calculatedto be 1.7%.

The ratio of the concentration of magnesium to fluorine was within therange of Mg:F=y:1 (3≤y≤5), more precisely Mg:F=about 3.9:1.

Next, the results of analyzing the bonding state of fluorine in thesample 7 after the second heating by the surface XPS analysis are shownin FIG. 39 . As a comparative example, the results of a sample preparedin the same manner as in the sample 7 except that 10 mol % LiF was usedas an addition starting material and no magnesium was added are shown.The XPS spectra of standard samples of MgF₂ and LiF are also shown.

As shown in FIG. 39 , in the sample in which 10 mol % LiF was used as anaddition starting material and no magnesium was added, the peak of thebinding energy of fluorine was about 685 eV which is equal to LiF, andit was considered that LiF is the main bonding state for fluorine on thesuperficial portion of the positive electrode active material. On theother hand, in the sample 7 having 1 mol % MgO and 2 mol % LiF added asthe starting material and having the second region, the peak of thebinding energy of fluorine on the superficial portion of the positiveelectrode active material was 682 eV or more and less than 685 eV, moreaccurately, was 684.3 eV, which was not consistent with either MgF₂ orLiF. In other words, fluorine contained in the second region of thepositive electrode active material was presumed to exist in a bondedstate not MgF₂ or LiF.

[Example 4]

In the present example, examination results of the temperature of thesecond heating and the atmosphere during the second heating when thepositive electrode active material having the second region formed bysegregation was formed will be described.

<<Temperature of Second Heating>> <Preparation of Positive ElectrodeActive Materials of Sample 11 to Sample 13>

Positive electrode active material of the sample 11 to the sample 13, inwhich the temperature of the second heating was changed, were prepared.For all starting materials, lithium carbonate and cobalt oxide were usedas common starting materials, and 1 mol % MgO and 2 mol % LiF were usedas addition starting materials.

The positive electrode active materials were prepared in the same manneras in the sample 7 of Example 1 except that the temperature of thesecond heating was 700° C. for the sample 11, 900° C. for the sample 12,and 1000° C. for the sample 13. Note that the temperature of the secondheating for the sample 7 was 800° C. The temperature of the secondheating for each sample is shown in Table 7.

TABLE 7 Addition starting Temperature of material second heating (° C.)Sample 11 1 mol % MgO  700 Sample 7 2 mol % LiF  800 Sample 12  900Sample 13 1000

Secondary batteries were prepared in the same manner as in Example 1,using the positive electrode active materials of the sample 7 and thesample 11 to the sample 13, and the cycle characteristics wereevaluated. The cycle characteristics of the sample 7 and the sample 11to sample 13 are shown in FIGS. 40A and 40B. The charge and dischargeconditions were the same as those in Example 1.

FIG. 40A is a graph of energy density at 4.6 V charging, and FIG. 40B isa graph of energy density retention rate at 4.6 V charging. As shown inFIG. 40B, the sample 7 with the second heating temperature of 800° C.showed the best cycle characteristics. The sample 11 at the secondheating temperature of 700° C. and the sample 12 at 900° C. had thesecond best cycle characteristics. Even in the sample 13 at which thesecond heating temperature of 1000° C., the energy density retentionrate after 20 cycles was 76%. This can be said that the sample 13exhibited favorable cycle characteristics as compared with the sample 6shown in FIG. 26 that had no addition starting material and had theenergy density retention rate of 63% after 20 cycles.

Accordingly, the temperature of the second heating is preferably 700° C.or more and 1000° C. or less, more preferably 700° C. or more and 900°C. or less, and much more preferably approximately 800° C.

<<Atmosphere of Second Heating>> <Preparation of Positive ElectrodeActive Material of Sample 14 to Sample 16>

Positive electrode active materials of the sample 14 to the sample 16were prepared by changing the atmosphere of the second heating from dryair to 100% oxygen. For all starting materials, lithium carbonate andcobalt oxide were used as common starting materials, and 1 mol % MgO and2 mol % LiF were used as addition starting materials. The positiveelectrode active materials were prepared in the same manner as thesample 7, the sample 12, and the sample 13, except that the secondheating was changed to an oxygen atmosphere.

Secondary batteries were fabricated in the same manner as in Example 1,using the positive electrode active materials of the sample 14 to thesample 16, and the cycle characteristics were evaluated together withthe samples 7, 12, and 13.

Graphs of energy density and cycle characteristics of the sample 7 andthe sample 12 to the sample 16 are shown in FIGS. 41A to 41C and FIGS.42A to 42C. FIGS. 41A to 41C are graphs of energy density, and FIGS. 42Ato 42C are graphs of cycle characteristics. FIGS. 41A and 42A show theenergy density and the cycle characteristics of the sample 14 and thesample 7 with the second heating temperature of 800° C. FIGS. 41B and42B show the energy density and the cycle characteristics of the sample15 and the sample 12 with the second heating temperature of 900° C.FIGS. 41C and 42C show the energy density and the cycle characteristicsof the sample 16 and the sample 13 with the second heating temperatureof 1000° C. The second heating atmosphere and the second heatingtemperature of each sample shown in FIGS. 41A to 41C and FIGS. 42A to42C are shown in Table 8.

TABLE 8 Temperature of Atmosphere second heating of second (° C.)heating Sample 14  800 O₂ Sample 7  800 Dry Air Sample 15  900 O₂ Sample12  900 Dry Air Sample 16 1000 O₂ Sample 13 1000 Dry Air

As shown in FIGS. 41A to 41C, when the second heating at 800° C., 900°C., and 1000° C. was performed, the second heating performed in anoxygen atmosphere contributed to better cycle characteristics than thatperformed in the dry air.

[Example 5]

In this example, the cycle characteristics when magnesium and fluorinewere used as addition starting materials and the cycle characteristicswhen an element other than magnesium or fluorine was used were compared.

<<Comparison Between Fluorine and Chlorine>>

First, the cycle characteristics were compared between the case of usingmagnesium and fluorine as the addition starting materials and the caseof using chlorine instead of fluorine.

<Preparation of Positive Electrode Active Material of Samples 17 and 18>

For the sample 7, 1 mol % MgO and 2 mol % LiF with respect to cobaltwere used as addition starting materials. For the sample 17, 1 mol %MgO, 1 mol % LiF, and 1 mol % LiCl were used. For the sample 18 as acomparative example, 1 mol % MgO and 2 mol % LiCl were used. For thesample 6 as a comparative example, neither magnesium nor fluorine norchlorine was added.

For the sample 7, the sample 17, the sample 18, and the sample 6,positive electrode active materials were prepared in the same manner asin Example 2, and secondary batteries using them were prepared, toevaluate the cycle characteristics. The cycle characteristic test wascarried out in the same manner as in Example 1.

<Cycle Characteristics>

Table 9 shows the addition starting materials and the energy densityretention rate after 20 cycles of each sample.

TABLE 9 Addition Energy density retention starting rate (%) at 4.6 Vcharging material after 20 cycles Sample 7 1 mol % MgO, 97.0 2 mol % LiFSample 17 1 mol % MgO, 80.2 1 mol % LiF, 1 mol % LiCl Sample 18 1 mol %MgO, 55.8 (compar- 2 mol % LiCl ative example) Sample 6 — 62.9 (compar-ative example)

As shown in Table 9, when chlorine was added in place of fluorine, cyclecharacteristics tended to decrease. However, in the sample 17 having 1mol % fluorine and 1 mol % chlorine, the energy density retention rateafter 20 cycles was 80% or more. This was a good cycle characteristic ascompared with the sample 6 having neither magnesium nor fluorine norchlorine.

<<Comparison Between Magnesium and Other Metals>>

Next, the cycle characteristics were compared between the case wheremagnesium and fluorine were used as addition starting materials and thecase where other metals were used instead of magnesium.

<Preparation of Positive Electrode Active Material of Sample 19 toSample 29>

The sample 7 of Example 1 was used as a sample using magnesium andfluorine as addition starting materials. For a sample 19 as acomparative example, 1 mol % MgO, 1 mol % TiO₂, and 2 mol % LiF wereused as addition starting materials. For a sample 20 as a comparativeexample, 1 mol % ZrO2 and 2 mol % LiF were used as addition startingmaterials. For a sample 21 as a comparative example, 1 mol % TiO₂ and 2mol % LiF were used as addition starting materials. For a sample 22 as acomparative example, 1 mol % V₂O₅ and 2 mol % LiF were used as additionstarting materials. For a sample 23 as a comparative example, 1 mol %ZnO and 2 mol % LiF were used as addition starting materials. For asample 24 as a comparative example, 1 mol % CaO and 2 mol % LiF wereused as addition starting materials. For a sample 25 as a comparativeexample, 1 mol % Al₂O₃ and 2 mol % LiF were used as addition startingmaterials. For a sample 26 as a comparative example, 1 mol % MoO₂ and 2mol % LiF were used as addition starting materials. For a sample 27 as acomparative example, 1 mol % SrO and 2 mol % LiF were used as additionstarting materials. For a sample 28 as a comparative example, 1 mol %NaF and 1 mol % LiF were used as addition starting materials. For asample 29 as a comparative example, 1 mol % BaO and 2 mol % LiF wereused as addition starting materials. As a comparative example in whichneither fluorine nor any metal was added, the sample 6 of Example 1 wasused.

For the sample 6, the sample 7, and the samples 19 to 29, positiveelectrode active materials were prepared in the same manner as inExample 1, and secondary batteries using them were prepared to evaluatecycle characteristics.

<Cycle Characteristics>

Table 10 shows the addition starting materials and the energy densityretention rate after 20 cycles of each sample.

TABLE 10 Energy density retention rate Addition (%) at 4.6 V startingcharging after material 20 cycles Sample 7 1 mol % MgO, 97.0 2 mol % LiFSample 19 1 mol % MgO, 58.5 (comparative 1 mol % TiO₂, example) 2 mol %LiF Sample 20 1 mol % 58.4 (comparative ZrO₂, example) 2 mol % LiFSample 21 1 mol % 55.2 (comparative TiO₂, 2 mol % example) LiF Sample 221 mol % V₂O₅, 54.8 (comparative 2 mol % LiF example) Sample 23 1 mol %ZnO, 53.8 (comparative 2 mol % LiF example) Sample 24 1 mol % CaO, 51.0(comparative 2 mol % LiF example) Sample 25 1 mol % 49.7 (comparativeAl₂O₃, example) 2 mol % LiF Sample 26 1 mol % 49.5 (comparative MoO₂,example) 2 mol % LiF Sample 27 1 mol % 49.0 (comparative SrO, 2 mol %LiF example) Sample 28 1 mol % NaF, 47.3 (comparative 1 mol % LiFexample) Sample 29 1 mol % 4.9 (comparative BaO, example) 2 mol % LiFSample 6 — 62.9 (comparative example)

As shown in Table 10, when other metals were added in place ofmagnesium, cycle characteristics tended to deteriorate.

From these results, it became clear that it is extremely effective wayto use magnesium and fluorine in combination as addition startingmaterials.

From the previous example, it was revealed that magnesium segregates onthe surface of the positive electrode active material by addingmagnesium and fluorine as starting materials of the positive electrodeactive material. Moreover, it became clear that the positive electrodeactive material can have high capacity and excellent cyclecharacteristics since it has a good coating layer formed by segregation.

A secondary battery having such a positive electrode active material hashigh capacity and long lifetime, so it is suitable for portableelectronic devices. Furthermore, when used to cars and other vehicles,it is also possible to avoid using commercial power at the peak ofelectric power demand, which can contribute to energy saving andreduction of carbon dioxide emissions.

[Example 6]

In the present example, evaluation results of a prepared positiveelectrode active material in which nickel, manganese, and cobalt areused as the transition metal of the first region will be described.

<Sample 31, Sample 32>

A sample 31 having magnesium and fluorine and a sample 32 having neithermagnesium nor fluorine as a comparative example were prepared.

The sample 31 was a sample obtained by adding 1 atomic % of magnesiumand 2 atomic % of fluorine with respect to the sum of nickel, manganese,and cobalt as starting materials. In addition, the atomic ratio betweennickel, manganese, and cobalt as starting materials was Ni:Mn:Co=1:1:1.

First, lithium carbonate (Li₂CO₃) was used as the lithium source of thecommon starting material. As the nickel source, nickel oxide (NiO) wasused. As the manganese source, manganese oxide (MnO₂) was used. As thecobalt source, cobalt oxide (Co₃O₄) was used. As the magnesium source ofthe addition starting material, magnesium oxide (MgO) was used. As thefluorine source, lithium fluoride (LiF) was used.

Each starting material was weighed so as to have an atomic ratio ofLiCo_(0.323)Mn_(0.333)Ni_(0.333)O₂+MgO_(0.01)LiF_(0.02).

Next, the weighed starting materials were mixed with a ball mill.

Then, the mixed starting materials were baked. The baking was performedat 950° C. for 10 hours under the following conditions: the temperaturerising rate was 200° C./h; and the flow rate of dry air atmosphere was10 L/min.

Through the above process, particles of composite oxide containinglithium, nickel, manganese, cobalt, fluorine, and magnesium weresynthesized.

The synthesized composite oxide particles were cooled to roomtemperature.

Then, the composite oxide particles were heated. The heating wasperformed in a dry air atmosphere under the following conditions: thetemperature was 800° C. (the temperature rising rate was 200° C./h); andthe retention time was two hours.

The heated particles were cooled to room temperature and subjected tocrushing treatment. In the crushing treatment, the particles were madeto pass through a sieve having an aperture width of 53 μm.

The particles which were subjected to the crushing treatment were usedas a positive electrode active material of the sample 31.

For the sample 32, each starting material was weighed to have an atomicratio of LiCo_(0.333)Mn_(0.333)Ni_(0.333)O₂. In addition, baking wasperformed at 1000° C. For other than that, the fabrication method of thesample 31 can be referred to.

Table 11 shows the fabrication conditions of the samples 31 and 32.

TABLE 11 Common starting Addition Mg:F material starting material(atomic ratio) Sample 31 Li₂CO₃ 1 mol % MgO, 1:2 NiO, MnO₂, 2 mol % LiFSample 32 Co₃O₄ — —

<Cycle Characteristics>

Next, CR2032 coin-type secondary batteries (with a diameter of 20 mm anda height of 3.2 mm) were fabricated using the positive electrode activematerials of the sample 31 and the sample 32 formed in the abovemanners. Their cycle characteristics were evaluated.

A positive electrode formed by applying slurry in which the positiveelectrode active materials of the samples 31 and 32, acetylene black(AB), and polyvinylidene fluoride (PVDF) were mixed at a weight ratio ofpositive electrode active material:AB:PVDF=95:2.5:2.5 to an aluminumfoil current collector was used. N-methyl-2-pyrrolidone (NMP) was usedas a solvent.

A lithium metal was used for the counter electrode.

As an electrolyte contained in an electrolyte solution, 1 mol/L lithiumhexafluorophosphate (LiPF₆) was used. As the electrolyte solution, asolution in which ethylene carbonate (EC) and diethyl carbonate (DEC) ata volume ratio of EC:DEC=3:7 and vinylene carbonate (VC) at a 2 weight %were mixed was used.

A positive electrode can and a negative electrode can were formed ofstainless steel (SUS).

The measurement temperature in the cycle characteristics test was 25° C.Charging was carried out at a constant current with a current density of68.5 mA/g per active material weight, and an upper limit voltage of 4.6V. After that, constant voltage charging was performed until the currentdensity reached 1.4 mA/g. Discharge was carried out at a constantcurrent with a current density of 68.5 mA/g per active material weight,with a lower limit voltage of 2.5 V.

FIG. 43A shows the discharge capacity of the secondary batteries usingthe positive electrode active materials of the sample 31 and the sample32 at 4.6 V charging. FIG. 43B shows the discharge capacity retentionrate thereof.

Compared to the sample 32 with no addition of magnesium or fluorine, thesample 31 with addition of magnesium and fluorine showed very good cyclecharacteristics.

Next, the results of various analyzes of the sample 31 are shown below.

<STEM-FFT>

STEM images of the cross section of the vicinity of the surface of thepositive electrode active material of the sample 31 are shown in FIGS.44A and 44B and FIGS. 45A and 45B. FIG. 44B is a STEM image obtained byenlarging a part of FIG. 44A. FIGS. 45A and 45B are HAADF-STEM imagesobtained by enlarging a part of FIG. 44A.

As is apparent from FIGS. 45A and 45B, in a region of about 0.5 nm fromthe surface of the positive electrode active material, a state where thebrightness is different from that in other regions was observed. This isprobably due to the large amount of magnesium which is a lighter elementthan the transition metal.

Also, in the region from about 0.5 nm to 5 nm from the surface of thepositive electrode active material, a state where the regularity wasdifferent from that in the internal region was observed. It wasconsidered that this is because the orientation of crystals differsbetween the region from about 0.5 nm to 5 nm from the surface and theinternal region.

FIG. 46A is a bright field STEM image in the same range as FIG. 45B.FIG. 46B shows a fast Fourier transform (FFT) image of a regionindicated by FFT1 in FIG. 46A. Some luminescent spot of the FFT1 imageis called A, B, C, O as shown in FIG. 46B.

Regarding the luminescent spots of the FFT image in the region indicatedby FFT1, the measured values were d=0.22 nm for OA, d=0.25 nm for OB,and d=0.23 nm for OC. In addition, ∠AOB=58°, ∠BOC=69°, ∠AOC=127°.

The results are close to d=0.21 nm for OA(200), d=0.24 nm for OB(1-11),d=0.24 nm for OC(-1-11), ∠AOB=55°, ∠BOC=70°, ∠AOC=125° which areobtained from magnesium oxide (MgO) data (ICDD45-0945) in the ICDD(International Centre for Diffraction Data). Therefore, it became clearthat the region indicated by FFT1 is a region having a rock salt typecrystal structure and is an image of [011] incidence.

FIG. 46C shows an FFT image of a region indicated by FFT2 in FIG. 46A.Some luminescent spot of the FFT2 image is called A, B, C, O as shown inFIG. 46C.

Regarding the luminescent spots of the FFT image in the region indicatedby FFT2, the measured values were d=0.25 nm for OA, d=0.21 nm for OB,and d=0.49 nm for OC. In addition, ∠AOB=26°, ∠BOC=57°, ∠AOC=83°.

The results are close to d=0.24 nm for OA(10-11), d=0.20 nm forOB(10-14), d=0.47 nm for OC(0003), ∠AOB=25°, ∠BOC=55°, ∠AOC=80° whichare obtained from lithium cobalt oxide (LiCoO₂) data (ICDD50-0653) inthe ICDD database. Therefore, it became clear that the region indicatedby FFT2 is a region having a layered rock-salt type crystal structureand is an image of [-12-10] incidence.

FIG. 46D shows an FFT image of a region indicated by FFT3 in FIG. 46A.Some luminescent spot of the FFT3 image is called A, B, C, O as shown inFIG. 46D.

Regarding the luminescent spots of the FFT image in the region indicatedby FFT3, the measured values were d=0.21 nm for OA, d=0.26 nm for OB,and d=0.24 nm for OC. In addition, ∠AOB=56°, ∠BOC=72°, ∠AOC=128°.

The results are close to d=0.20 nm for OA(01-14), d=0.23 nm forOB(10-1-2), d=0.23 nm for OC(1-102), ∠AOB=55°, ∠BOC=70°, ∠AOC=125° whichare obtained from lithium cobalt oxide (LiCoO₂) data (ICDD50-0653) inthe ICDD database. Therefore, it became clear that the region indicatedby FFT2 is a region having a layered rock-salt type crystal structureand is an image of [02-21] incidence.

In other words, the regions indicated by FFT2 and FFT3 were found tohave the same layered rock-salt type crystal structure, but to havedifferent crystal axis directions from each other.

In the range observable in FIG. 45A, FIG. 45B, and FIG. 46A, it wasobserved that the crystal orientations roughly coincided with each othereven if the brightness was different.

FIG. 47 shows the structure near the surface of the positive electrodeactive material, which is estimated from the STEM-FFT result, togetherwith the STEM image. M in FIG. 47 indicates any one of nickel,manganese, and cobalt.

FFT3 which is a region in an inner portion of the positive electrodeactive material has a layered rock-salt type crystal structure. Also, itis an image of [02-21] incidence observed when lithium and M atomsoverlap with each other.

In addition, FFT2 which is a region of the superficial portion of thepositive electrode active material has a layered rock-salt type crystalstructure. FFT2 is an image of [-12-10] incidence showing the state ofrepeating the layer of oxygen atoms, the layer of M (one of nickel,manganese, and cobalt) atoms, and the layer of lithium atoms. It isconsidered that the darker layer and the brighter layer are repeated inthe bright field STEM image because the layer of M, and the layer ofoxygen and lithium are repeated. That is, although FFT3 and FFT2 havethe same layered rock-salt type crystal structure, the crystal axisdirections are different from each other.

Also, in the region of the superficial portion of the positive electrodeactive material, FFT1 which is a region closer to the surface than FFT2has a rock-salt type crystal structure and is an image of [011]incidence.

<EDX>

Next, the cross section of the positive electrode active material of thesample 31 near the surface was analyzed using EDX, and the results areshown in FIGS. 48A1 to 48B2 and FIG. 49A1 to B2.

FIG. 48A1 is a HAADF-STEM image, FIG. 48A2 is a mapping of oxygen, FIG.48B1 is a mapping of magnesium, and FIG. 48B2 is a mapping of fluorine.FIG. 49A1 is the same HAADF-STEM image as in FIG. 48A1, FIG. 49A2 is amapping of manganese, FIG. 49B1 is a mapping of nickel, and FIG. 49B2 isa mapping of cobalt.

From FIG. 48B1, it was observed that magnesium segregated in the regionof about 3 nm from the surface of the positive electrode activematerial. By comparing FIGS. 49A2, 49B1, and 49B2, it was observed thatthere was a region containing less manganese and more nickel and cobaltthan the inner side, on where the superficial portion of the positiveelectrode active material. This region was about 5 nm from the surfaceand almost overlapped with the region in which regularity different fromthe inner portion was observed in the STEM image.

Therefore, it was confirmed that the positive electrode active materialof the sample 31 is a positive electrode active material having a regioncontaining magnesium in the superficial portion and a region having alow content of manganese in a part of the inner portion.

From the above results, it became clear that the positive electrodeactive material of the sample 31, which was prepared by setting themolar ratio of starting materials to LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂+1 mol% MgO+2 mol % LiF and heating at 800° C., has the followingcharacteristics.

First, the positive electrode active material of the sample 31 includesa second region having magnesium oxide in the superficial portion. Inthe inner portion of the positive electrode active material, a regioncontaining LiNi_(x)Mn_(y)Co_(z)O₂ (x+y+z=1) having a layered rock-salttype crystal structure at a part closer to the center part, and a regioncontaining LiNi_(a)Mn_(b)Co_(c)O₂ (a+b+c=1) having a layered rock-salttype crystal structure at a part closer to the surface are included.

The internal LiNi_(x)Mn_(y)Co_(z)O₂ and LiNi_(a)Mn_(b)Co_(c)O₂ have thesame layered rock-salt type crystal structure, but the crystal axisdirections may be different in some cases.

The content of each element is y>b, and the content of manganese withrespect to the sum of nickel, manganese, and cobalt may be low in theregion close to the surface.

The positive electrode active material of the sample 31 having the abovecharacteristics exhibits extremely favorable cycle characteristics whenused for a secondary battery.

[Example 7]

In the present example, analysis results of a positive electrode activematerial prepared by adding cobalt as a transition metal and addingmagnesium and fluorine as starting materials using EELS will bedescribed.

The sample 7 of Example 1 using 1 mol % MgO and 2 mol % LiF as additionstarting materials was used as an analysis sample of the presentexample.

The state of cobalt at the six analysis points from *1 to *6 in a crosssection of the sample 7 was analyzed using EELS. FIG. 50 is a STEM imageof the cross section of the vicinity of the surface of the positiveelectrode active material of the sample 7 used for EELS analysis, inwhich analysis points of *1 (the depth from the surface is about 1 nm),*2 (about 2.5 nm), and *3 (about 5 nm) are shown. Furthermore, *4 isabout 10 nm from the surface of the positive electrode active material,*5 is about 100 nm from the surface of the positive electrode activematerial, and *6 is near the center of the particle of the positiveelectrode active material.

Table 12 and FIG. 51 show the EELS spectral intensity ratios of the L₂level and the L₃ level of cobalt in each analysis point. The higher theL₃/L₂ is, the lower the valence of cobalt becomes.

TABLE 12 Measurement L₃/ region L₂ *1 4.6 *2 3.0 *3 3.2 *4 3.0 *5 2.9 *63.1

As is apparent from Table 12 and FIG. 51 , L₃/L₂ of the analysis point*1 closest to the surface of the positive electrode active material wasthe highest, 4.6. From the analysis point *2 to the analysis point *6,L₃/L₂ was lower than that of the analysis point *1 and was within therange from 2.9 to 3.2 and no large difference was observed.

From these results, it was inferred that the amount of divalent cobaltexisting as cobalt oxide (CoO) was large in the analysis point *1, andthat the amount of trivalent cobalt existing as lithium cobalt oxide(LiCoO₂) was large in the analysis points *2 to *6.

REFERENCE NUMERALS

100: positive electrode active material, 101: first region, 102: secondregion, 103: third region, 200: active material layer, 201: graphenecompound, 211 a: positive electrode, 211 b: negative electrode, 212 a:lead, 212 b: lead, 214: separator, 215 a: bonding portion, 215 b:bonding portion, 217: fixing member, 250: secondary battery, 251:exterior body, 261: folded portion, 262: seal portion, 263: sealportion, 271: crest line, 272: trough line, 273: space, 300: secondarybattery, 301: positive electrode can, 302: negative electrode can, 303:gasket, 304: positive electrode, 305: positive electrode currentcollector, 306: positive electrode active material layer, 307: negativeelectrode, 308: negative electrode current collector, 309: negativeelectrode active material layer, 310: separator, 500: secondary battery,501: positive electrode current collector, 502: positive electrodeactive material layer, 503: positive electrode, 504: negative electrodecurrent collector, 505: negative electrode active material layer, 506:negative electrode, 507: separator, 508: electrolyte solution, 509:exterior body, 510: positive electrode lead electrode, 511: negativeelectrode lead electrode, 600: secondary battery, 601: positiveelectrode cap, 602:

battery can, 603: positive electrode terminal, 604: positive electrode,605: separator, 606: negative electrode, 607: negative electrodeterminal, 608: insulating plate, 609: insulating plate, 611: PTCelement, 612: safety valve mechanism, 900: circuit board, 910: label,911: terminal, 912: circuit, 913: secondary battery, 914: antenna, 915:sealant, 916: layer, 917: layer, 918: antenna, 920: display device, 921:sensor, 922: terminal, 930: housing, 930 a: housing, 930 b: housing,931: negative electrode, 932: positive electrode, 933: separator, 950:wound body, 951: terminal, 952: terminal, 980: secondary battery, 993:wound body, 994: negative electrode, 995: positive electrode, 996:separator, 997: lead electrode, 998: lead electrode, 1001: crystaldefect, 7100: portable display device, 7101: housing, 7102: displayportion, 7103: operation button, 7104: secondary battery, 7200: portableinformation terminal, 7201: housing, 7202: display portion, 7203: band,7204: buckle, 7205: operation button, 7206: input output terminal, 7207:icon, 7300: display device, 7304: display portion, 7400: mobile phone,7401: housing, 7402: display portion, 7403: operation button, 7404:external connection port, 7405: speaker, 7406: microphone, 7407:secondary battery, 7408: lead electrode, 7409: current collector, 8000:display device, 8001: housing, 8002: display portion, 8003: speakerportion, 8004: secondary battery, 8021: charging apparatus, 8022: cable,8024: secondary battery, 8100: lighting device, 8101: housing, 8102:light source, 8103: secondary battery, 8104: ceiling, 8105: wall, 8106:floor, 8107: window, 8200: indoor unit, 8201: housing, 8202: air outlet,8203: secondary battery, 8204: outdoor unit, 8300: electricrefrigerator-freezer, 8301: housing, 8302: refrigerator door, 8303:freezer door, 8304: secondary battery, 8400: automobile, 8401:headlight, 8406: electric motor, 8500: automobile, 8600: motor scooter,8601: side mirror, 8602: secondary battery, 8603: indicator, 8604:storage unit under seat, 9600: tablet terminal, 9625: switch, 9626:switch, 9627: power switch, 9628: operation switch, 9629: fastener,9630: housing, 9630 a: housing, 9630 b: housing, 9631: display portion,9633: solar cell, 9634: charge and discharge control circuit, 9635:power storage unit, 9636: DC-DC converter, 9637: converter, 9640:movable portion.

This application is based on Japanese Patent Application Serial No.2016-200835 filed with Japan Patent Office on Oct. 12, 2016, JapanesePatent Application Serial No. 2017-052309 filed with Japan Patent Officeon Mar. 17, 2017, and Japanese Patent Application Serial No. 2017-100619filed with Japan Patent Office on May 22, 2017, the entire contents ofwhich are hereby incorporated by reference.

1. A method for forming a lithium-ion secondary battery comprising anelectrolyte solution, a negative electrode, and a positive electrodecomprising a positive electrode active material, the method comprisingthe steps of: mixing a lithium source, a cobalt source, a magnesiumsource, and a fluorine source to form a mixture, performing a firstheating at a temperature higher than or equal to 800° C. and lower thanor equal to 1050° C. of the mixture whereby a composite oxide issynthesized, performing a second heating at a temperature higher than orequal to 500° C. and lower than or equal to 1200° C. of the compositeoxide whereby the positive electrode active material is formed, whereinmagnesium in the magnesium source and fluorine in the fluorine sourceare segregated to a superficial portion of the positive electrode activematerial by the heating, wherein the positive electrode active materialcomprises a layered rock-salt crystal structure in an inner portion ofthe positive electrode active material, wherein the positive electrodeactive material comprises a rock-salt crystal structure in thesuperficial portion of the positive electrode active material, whereinthe positive electrode active material comprises lithium cobalt oxide inthe inner portion of the positive electrode active material, wherein thepositive electrode active material comprises oxygen, cobalt, magnesium,and fluorine in the superficial portion of the positive electrode activematerial, wherein the superficial portion of the positive electrodeactive material comprises a region where a distribution of fluorineoverlaps with a distribution of magnesium, wherein a solution in whichethylene carbonate, diethyl carbonate, and vinylene carbonate are mixedis used as the electrolyte solution, wherein an electrolyte in theelectrolyte solution comprises lithium hexafluorophosphate, and whereinthe positive electrode active material comprises an electrolytedecomposition product in contact with at least part of the superficialportion.
 2. A method for forming a lithium-ion secondary batterycomprising an electrolyte solution, a negative electrode, and a positiveelectrode comprising a positive electrode active material, the methodcomprising the steps of: mixing a lithium source, a cobalt source, amagnesium source, and a fluorine source to form a mixture, performing afirst heating at a temperature higher than or equal to 800° C. and lowerthan or equal to 1050° C. of the mixture whereby a composite oxide issynthesized, performing a second heating at a temperature higher than orequal to 500° C. and lower than or equal to 1200° C. of the compositeoxide whereby the positive electrode active material is formed, whereinmagnesium in the magnesium source and fluorine in the fluorine sourceare segregated to a superficial portion of the positive electrode activematerial by the heating, wherein the positive electrode active materialcomprises a layered rock-salt crystal structure in an inner portion ofthe positive electrode active material, wherein the positive electrodeactive material comprises a rock-salt crystal structure in thesuperficial portion of the positive electrode active material, whereinthe positive electrode active material comprises lithium cobalt oxide inthe inner portion of the positive electrode active material, wherein thepositive electrode active material comprises oxygen, cobalt, magnesium,and fluorine in the superficial portion of the positive electrode activematerial, wherein the superficial portion of the positive electrodeactive material comprises a region where a distribution of fluorineoverlaps with a distribution of magnesium, wherein the electrolytesolution comprises vinylene carbonate, and wherein the positiveelectrode active material comprises an electrolyte decomposition productin contact with at least part of the superficial portion.
 3. A methodfor forming a lithium-ion secondary battery comprising an electrolytesolution, a negative electrode, and a positive electrode comprising apositive electrode active material, the method comprising the steps of:mixing a lithium source, a cobalt source, a magnesium source, and afluorine source to form a mixture, performing a first heating at atemperature higher than or equal to 800° C. and lower than or equal to1050° C. to the mixture whereby a composite oxide is synthesized,performing a second heating is at a temperature higher than or equal to500° C. and lower than or equal to 1200° C. of the composite oxidewhereby the positive electrode active material is formed, whereinmagnesium in the magnesium source and fluorine in the fluorine sourceare segregated to a superficial portion of the positive electrode activematerial by the heating, wherein the positive electrode active materialcomprises a layered rock-salt crystal structure in an inner portion ofthe positive electrode active material, wherein the positive electrodeactive material comprises a rock-salt crystal structure in thesuperficial portion of the positive electrode active material, whereinthe positive electrode active material comprises lithium cobalt oxide inthe inner portion of the positive electrode active material, wherein thepositive electrode active material comprises oxygen, cobalt, magnesium,and fluorine in the superficial portion of the positive electrode activematerial, wherein the superficial portion of the positive electrodeactive material comprises a region where a distribution of fluorineoverlaps with a distribution of magnesium, and wherein the positiveelectrode active material comprises an electrolyte decomposition productin contact with at least part of the superficial portion.
 4. The methodfor forming a lithium-ion secondary battery, according to claim 1,wherein the positive electrode active material comprises CoO(II),fluorine, and magnesium oxide having a rock-salt crystal structure inthe superficial portion of the positive electrode active material. 5.The method for forming a lithium-ion secondary battery, according toclaim 1, wherein the second heating is performed in an air atmosphere oran oxygen atmosphere.
 6. The method for forming a lithium-ion secondarybattery, according to claim 1, wherein a concentration ratio betweenmagnesium and fluorine in starting materials of the positive electrodeactive material is in a range of Mg:F=1:x (1.5≤x≤4) (atomic ratio). 7.The method for forming a lithium-ion secondary battery, according toclaim 1, wherein the electrolyte solution further comprises one or bothof adiponitrile and propane sultone.
 8. The method for forming alithium-ion secondary battery, according to claim 1, wherein theelectrolyte solution comprises ethylene carbonate and diethyl carbonateat a volume ratio of 3:7 and vinylene carbonate at a 2 weight %.
 9. Themethod for forming a lithium-ion secondary battery, according to claim1, wherein the second heating is performed at a temperature higher thanor equal to 700° C. and lower than or equal to 1000° C.
 10. The methodfor forming a lithium-ion secondary battery, according to claim 1,wherein the magnesium source comprises magnesium fluoride.
 11. Themethod for forming a lithium-ion secondary battery, according to claim1, wherein the fluorine source comprises lithium fluoride or magnesiumfluoride.
 12. The method for forming a lithium-ion secondary battery,according to claim 1, wherein a crystal orientation of the layeredrock-salt crystal structure in the inner portion of the positiveelectrode active material and a crystal orientation of the rock-saltcrystal structure in the superficial portion of the positive electrodeactive material are substantially aligned with each other.
 13. Themethod for forming a lithium-ion secondary battery, according to claim2, wherein the positive electrode active material comprises CoO(II),fluorine, and magnesium oxide having a rock-salt crystal structure inthe superficial portion of the positive electrode active material. 14.The method for forming a lithium-ion secondary battery, according toclaim 2, wherein the second heating is performed in an air atmosphere oran oxygen atmosphere.
 15. The method for forming a lithium-ion secondarybattery, according to claim 2, wherein a concentration ratio betweenmagnesium and fluorine in starting materials of the positive electrodeactive material is in a range of Mg:F=1:x (1.5≤x≤4) (atomic ratio). 16.The method for forming a lithium-ion secondary battery, according toclaim 2, wherein the electrolyte solution further comprises one or bothof adiponitrile and propane sultone.
 17. The method for forming alithium-ion secondary battery, according to claim 2, wherein theelectrolyte solution comprises ethylene carbonate and diethyl carbonateat a volume ratio of 3:7 and vinylene carbonate at a 2 weight %.
 18. Themethod for forming a lithium-ion secondary battery, according to claim2, wherein the second heating is performed at a temperature higher thanor equal to 700° C. and lower than or equal to 1000° C.
 19. The methodfor forming a lithium-ion secondary battery, according to claim 2,wherein the magnesium source comprises magnesium fluoride.
 20. Themethod for forming a lithium-ion secondary battery, according to claim2, wherein the fluorine source comprises lithium fluoride or magnesiumfluoride.
 21. The method for forming a lithium-ion secondary battery,according to claim 2, wherein a crystal orientation of the layeredrock-salt crystal structure in the inner portion of the positiveelectrode active material and a crystal orientation of the rock-saltcrystal structure in the superficial portion of the positive electrodeactive material are substantially aligned with each other.
 22. Themethod for forming a lithium-ion secondary battery, according to claim3, wherein the positive electrode active material comprises CoO(II),fluorine, and magnesium oxide having a rock-salt crystal structure inthe superficial portion of the positive electrode active material. 23.The method for forming a lithium-ion secondary battery, according toclaim 3, wherein the second heating is performed in an air atmosphere oran oxygen atmosphere.
 24. The method for forming a lithium-ion secondarybattery, according to claim 3, wherein a concentration ratio betweenmagnesium and fluorine in starting materials of the positive electrodeactive material is in a range of Mg:F=1:x (1.5≤x≤4) (atomic ratio). 25.The method for forming a lithium-ion secondary battery, according toclaim 3, wherein the electrolyte solution further comprises one or bothof adiponitrile and propane sultone.
 26. The method for forming alithium-ion secondary battery, according to claim 3, wherein theelectrolyte solution comprises ethylene carbonate and diethyl carbonateat a volume ratio of 3:7 and vinylene carbonate at a 2 weight %.
 27. Themethod for forming a lithium-ion secondary battery, according to claim3, wherein the second heating is performed at a temperature higher thanor equal to 700° C. and lower than or equal to 1000° C.
 28. The methodfor forming a lithium-ion secondary battery, according to claim 3,wherein the magnesium source comprises magnesium fluoride.
 29. Themethod for forming a lithium-ion secondary battery, according to claim3, wherein the fluorine source comprises lithium fluoride or magnesiumfluoride.
 30. The method for forming a lithium-ion secondary battery,according to claim 3, wherein a crystal orientation of the layeredrock-salt crystal structure in the inner portion of the positiveelectrode active material and a crystal orientation of the rock-saltcrystal structure in the superficial portion of the positive electrodeactive material are substantially aligned with each other.